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

	Fixes [Bug #21286]

This is a C API for extensions to resolve and get function symbols of other extensions.
Extensions can check the expected symbol is correctly loaded and accessible, and
use it if it is available.
Otherwise, extensions can raise their own error to guide users to setup their
environments correctly and what's missing.

Instead of `RUBY_REFERENCES_START` and `RUBY_REFERENCES_END`, so that
auto-indent works well.

Also move `struct` so that `typedef`-ed names can be used.