[PATCH] Remove rbimpl_rstring_getmem() usage as workaround for GCC 15.2.1 optimization bug. [Bug #21655]

	[PATCH] include/ruby/internal/core/rstring.h: Remove rbimpl_rstring_getmem() definition.

	[PATCH] simplify RSRING_GETMEM() definition.

	[PATCH] [Bug #21629] Enable `nonstring` attribute on clang 21

	[PATCH] [Bug #21629] Initialize `struct RString`

	[PATCH] [Bug #21629] Initialize `struct RArray`

	Suppress gcc 15 unterminated-string-initialization warnings

	Drop an ignored attribute
	MIME-Version: 1.0
	Content-Type: text/plain; charset=UTF-8
	Content-Transfer-Encoding: 8bit

	GCC 13.3.0 (Ubuntu 24.04) emits the following warning:

	../symbol.c: In function ‘rb_id_attrset’:
	../symbol.c:175:9: warning: ‘nonstring’ attribute ignored on objects of type ‘const char[][8]’ [-Wattributes]
	  175 |         RBIMPL_ATTR_NONSTRING() static const char id_types[][8] = {
	      |         ^~~~~~~~~~~~~~~~~~~~~

	Separate `__has_attribute` from `defined(__has_attribute)`

	Fix Visual C warnings:
	```
	regenc.h(121): warning C4067: unexpected tokens following preprocessor directive - expected a newline
	```

	Cast up `int` instruction code to `VALUE`

	Fix Visual C warnings:
	```
	iseq.c(3793): warning C4312: 'type cast': conversion from 'int' to 'void *' of greater size
	iseq.c(3794): warning C4312: 'type cast': conversion from 'int' to 'void *' of greater size
	```

	Do not let files depend on a phony target

	Detect `clock_gettime` and `clock_getres` for winpthreads

	Include stdbool.h without checking with autoconf

	As reported in <https://bugs.ruby-lang.org/issues/21340>, older autoconf
	have an AC_HEADER_STDBOOL that's incompatible with C23. Autoconf 2.72
	fixed the macro, but also mentions that it's obsolescent since all
	current compilers have this header.

	Since we require C99 [1] and VS 2015 [2], we might actually be able take
	that suggestion and include stdbool.h without a check. I want to try
	this on rubyci.org and will revert if this cause any issues. Not
	touching AC_HEADER_STDBOOL in configure.ac for now.

	[1]: https://bugs.ruby-lang.org/issues/15347
	[2]: https://bugs.ruby-lang.org/issues/19982

	Fix redefinition of `clock_gettime` and `clock_getres`

	winpthreads-git 12.0.0.r720 provides `clock_gettime` and
	`clock_getres` as inline functions.

	Detect `clock_gettime` and `clock_getres` for winpthreads

	Fix C23 (GCC 15) WIN32 compatibility for rb_define_* functions

	Fixes [Bug #21286]

Fix memory leak in Regexp capture group when timeout

[Bug #20650]

The capture group allocates memory that is leaked when it times out.

For example:

    re = Regexp.new("^#{"(a*)" * 10_000}x$", timeout: 0.000001)
    str = "a" * 1000000 + "x"

    10.times do
      100.times do
        re =~ str
      rescue Regexp::TimeoutError
      end

      puts `ps -o rss= -p #{$$}`
    end

Before:

    34688
    56416
    78288
    100368
    120784
    140704
    161904
    183568
    204320
    224800

After:

    16288
    16288
    16880
    16896
    16912
    16928
    16944
    17184
    17184
    17200

	Check if macros are defined before using

	Assume macros with the same prefix would be defined together.

	Refer autoconfigured endian macro (#10572)

	Remove the case `RB_IO_BUFFER_HOST_ENDIAN` is not defined.

* Avoiding using `Tempfile` which was retaining the file preventing it from unlinking.

* Restore experimental warnings.

* Documentation and code structure improvements.

* Improved validation of flags, clarified documentation of argument handling.

* Remove inconsistent use of `Example:` and add example to `null?`.

* Expose `IO::Buffer#private?` and add test.