atime may not updated unless strictatime is set on macOS Catalina

	Cited from mount(8):

	```
	strictatime
	        Always update the file access time when reading from a
	        file. Without this option the filesystem may default to a
	        less strict update mode, where some access time updates
	        are skipped for performance reasons. This option could be
	        ignored if it is not supported by the filesystem.
	```

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_6@67828 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

	fix VC 2013 compile error

	It seems the compiler does not support VLAs.
	See also: https://ci.appveyor.com/project/ruby/ruby/builds/26392589/job/px6nuiuw4e78weg1

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_6@67750 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

	fix tracepoint + backtrace SEGV

	PC modification in gc_event_hook_body was careless.  There are (so
	to say) abnormal iseqs stored in the cfp.  We have to check sanity
	before we touch the PC.

	This has not been fixed because there was no way to (ab)use the
	setup from pure-Ruby.  However by using our official C APIs it is
	possible to touch such frame(s), resulting in SEGV.

	Fixes [Bug #14834].

	Fix assertion failure when VM_CHECK_MODE

	Some VM frames (dummy and top pushed by `rb_vm_call_cfunc`) has
	iseq but has no pc.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_6@67745 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

	Get rid of indirect sharing

	* string.c (str_duplicate): share the root shared string if the
	  original string is already sharing, so that all shared strings
	  refer the root shared string directly.  indirect sharing can
	  cause a dangling pointer.

	[Bug #15792]

	str_duplicate: Don't share with a frozen shared string

	This is a follow up for 3f9562015e651735bfc2fdd14e8f6963b673e22a.
	Before this commit, it was possible to create a shared string which
	shares with another shared string by passing a frozen shared string
	to `str_duplicate`.

	Such string looks like:

	```
	 --------                    -----------------
	 | root | ------ owns -----> | root's buffer |
	 --------                    -----------------
	     ^                             ^   ^
	 -----------                       |   |
	 | shared1 | ------ references -----   |
	 -----------                           |
	     ^                                 |
	 -----------                           |
	 | shared2 | ------ references ---------
	 -----------
	```

	This is bad news because `rb_fstring(shared2)` can make `shared1`
	independent, which severs the reference from `shared1` to `root`:

	```c
	/* from fstr_update_callback() */
	str = str_new_frozen(rb_cString, shared2);  /* can return shared1 */
	if (STR_SHARED_P(str)) { /* shared1 is also a shared string */
	    str_make_independent(str);  /* no frozen check */
	}
	```

	If `shared1` was the only reference to `root`, then `root` can be
	reclaimed by the GC, leaving `shared2` in a corrupted state:

	```
	 -----------                         --------------------
	 | shared1 | -------- owns --------> | shared1's buffer |
	 -----------                         --------------------
	      ^
	      |
	 -----------                         -------------------------
	 | shared2 | ------ references ----> | root's buffer (freed) |
	 -----------                         -------------------------
	```

	Here is a reproduction script for the situation this commit fixes.

	```ruby
	a = ('a' * 24).strip.freeze.strip
	-a
	p a
	4.times { GC.start }
	p a
	```

	 - string.c (str_duplicate): always share with the root string when
	   the original is a shared string.
	 - test_rb_str_dup.rb: specifically test `rb_str_dup` to make
	   sure it does not try to share with a shared string.

	[Bug #15792]

	Closes: https://github.com/ruby/ruby/pull/2159

	Update dependencies


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_6@67731 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

	numeric.c: fix infinite loop

	* numeric.c (int_pow): fix infinite loop in the case of y equal 1
	  and power of x does not overflow.
	  [ruby-core:91734] [Bug #15651]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_6@67222 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@66485 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@66371 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@64495 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@64344 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

* include/ruby/ruby.h (UNREACHABLE_RETURN): UNREACHABLE at the end
  of non-void functions.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@64025 b2dd03c8-39d4-4d8f-98ff-823fe69b080e