Suppress gcc 15 unterminated-string-initialization warnings

	Drop an ignored attribute
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	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

3c471149910d79745b50389c436f0ed2973e0d91 added a call to `reset_sigmask`
but the guard condition didn't match with the condition used to define
`reset_sigmask`.

On Linux, while the signal handler runs, that signal is masked, so in
the rb_bug_for_fatal_signal() code path we didn't get the default signal
action as intended. See signal(7). It worked fine on macOS, though.

Before:

    $ ./miniruby -e 'Process.kill :SIGSEGV, Process.pid'
    <snip>
    Aborted (core dumped)

After:

    $ ./miniruby -e 'Process.kill :SIGSEGV, Process.pid'
    <snip>
    Segmentation fault (core dumped)

Follow-up for 1ac0afab4da "rb_bug_for_fatal_signal: exit with the right
signal".

When aborting by a sanitizer, `SIGILL` or something is raised and
often results in raising the same signal infinitely.

* Remove unused SIGCHLD handling.

* Remove unused `init_sigchld`.

* Remove unnecessary `#define RUBY_SIGCHLD (0)`.

* Remove unused `SIGCHLD_LOSSY`.