v.cflag: support #flag $when_first_existing(libABC.a, /some/path/libABC.a, ...), without panicing (unlike #flag $first_existing(...))

[spytheman]

This PR provides a way to cleanup vlib/sync/stdatomic/1.declarations.c.v later.
It will look like this:

$if linux {
	$if tinyc {
		$if amd64 {
			// most Linux distributions have /usr/lib/libatomic.so,
			// but Ubuntu uses gcc version specific dir
			#flag $when_first_existing('/usr/lib/gcc/x86_64-linux-gnu/6/libatomic.a','/usr/lib/gcc/x86_64-linux-gnu/7/libatomic.a','/usr/lib/gcc/x86_64-linux-gnu/8/libatomic.a','/usr/lib/gcc/x86_64-linux-gnu/9/libatomic.a','/usr/lib/gcc/x86_64-linux-gnu/10/libatomic.a','/usr/lib/gcc/x86_64-linux-gnu/11/libatomic.a','/usr/lib/gcc/x86_64-linux-gnu/12/libatomic.a','/usr/lib/gcc/x86_64-linux-gnu/13/libatomic.a','/usr/lib/gcc/x86_64-linux-gnu/14/libatomic.a')
			// Redhat/CentOS paths:
			#flag $when_first_existing('/usr/lib/gcc/x86_64-redhat-linux/6/libatomic.a','/usr/lib/gcc/x86_64-redhat-linux/7/libatomic.a','/usr/lib/gcc/x86_64-redhat-linux/8/libatomic.a','/usr/lib/gcc/x86_64-redhat-linux/9/libatomic.a','/usr/lib/gcc/x86_64-redhat-linux/10/libatomic.a','/usr/lib/gcc/x86_64-redhat-linux/11/libatomic.a','/usr/lib/gcc/x86_64-redhat-linux/12/libatomic.a','/usr/lib/gcc/x86_64-redhat-linux/13/libatomic.a','/usr/lib/gcc/x86_64-redhat-linux/14/libatomic.a')
			// Gentoo paths:
			#flag $when_first_existing('/usr/lib/gcc/x86_64-pc-linux-gnu/6/libatomic.a','/usr/lib/gcc/x86_64-pc-linux-gnu/7/libatomic.a','/usr/lib/gcc/x86_64-pc-linux-gnu/8/libatomic.a','/usr/lib/gcc/x86_64-pc-linux-gnu/9/libatomic.a','/usr/lib/gcc/x86_64-pc-linux-gnu/10/libatomic.a','/usr/lib/gcc/x86_64-pc-linux-gnu/11/libatomic.a','/usr/lib/gcc/x86_64-pc-linux-gnu/12/libatomic.a','/usr/lib/gcc/x86_64-pc-linux-gnu/13/libatomic.a','/usr/lib/gcc/x86_64-pc-linux-gnu/14/libatomic.a')
			// OpenSUSE paths:
			#flag $when_first_existing('/usr/lib64/gcc/x86_64-suse-linux/6/libatomic.a','/usr/lib64/gcc/x86_64-suse-linux/7/libatomic.a','/usr/lib64/gcc/x86_64-suse-linux/8/libatomic.a','/usr/lib64/gcc/x86_64-suse-linux/9/libatomic.a','/usr/lib64/gcc/x86_64-suse-linux/10/libatomic.a','/usr/lib64/gcc/x86_64-suse-linux/11/libatomic.a','/usr/lib64/gcc/x86_64-suse-linux/12/libatomic.a','/usr/lib64/gcc/x86_64-suse-linux/13/libatomic.a','/usr/lib64/gcc/x86_64-suse-linux/14/libatomic.a')
			// ALT Linux paths:
			#flag $when_first_existing('/usr/lib64/gcc/x86_64-alt-linux/6/libatomic.a','/usr/lib64/gcc/x86_64-alt-linux/7/libatomic.a','/usr/lib64/gcc/x86_64-alt-linux/8/libatomic.a','/usr/lib64/gcc/x86_64-alt-linux/9/libatomic.a','/usr/lib64/gcc/x86_64-alt-linux/10/libatomic.a','/usr/lib64/gcc/x86_64-alt-linux/11/libatomic.a','/usr/lib64/gcc/x86_64-alt-linux/12/libatomic.a','/usr/lib64/gcc/x86_64-alt-linux/13/libatomic.a','/usr/lib64/gcc/x86_64-alt-linux/14/libatomic.a')
			$if musl ? {
				#flag $when_first_existing('/usr/lib/gcc/x86_64-pc-linux-musl/6/libatomic.a','/usr/lib/gcc/x86_64-pc-linux-musl/7/libatomic.a','/usr/lib/gcc/x86_64-pc-linux-musl/8/libatomic.a','/usr/lib/gcc/x86_64-pc-linux-musl/9/libatomic.a','/usr/lib/gcc/x86_64-pc-linux-musl/10/libatomic.a','/usr/lib/gcc/x86_64-pc-linux-musl/11/libatomic.a','/usr/lib/gcc/x86_64-pc-linux-musl/12/libatomic.a','/usr/lib/gcc/x86_64-pc-linux-musl/13/libatomic.a','/usr/lib/gcc/x86_64-pc-linux-musl/14/libatomic.a')
			}
		}
	} $else {
		#flag -latomic
	}
}

With it, the passed arguments to tcc will not be as long anymore, since they will not include paths for linux distros that are != the current one.

Here is an example of the generated command passed to tcc, with #flag $when_first_existing():

#0 13:39:49 ^ support_flag_when_first_existing ~/code/v>./v -no-retry-compilation -cc tcc -showcc /home/delian/code/misc/2025_02_21__13/x.c.v
> C compiler cmd: '/home/delian/code/v/thirdparty/tcc/tcc.exe' '@/tmp/v_1000/x.01JMM50PDMPJTAZZRW71EKJ6T0.tmp.c.rsp'
> C compiler response file "/tmp/v_1000/x.01JMM50PDMPJTAZZRW71EKJ6T0.tmp.c.rsp":
  -fwrapv -o "/home/delian/code/misc/2025_02_21__13/x" -D GC_THREADS=1 -D GC_BUILTIN_ATOMIC=1 -I "/home/delian/code/v/thirdparty/libgc/include" "/tmp/v_1000/x.01JMM50PDMPJTAZZRW71EKJ6T0.tmp.c" -std=gnu99 -D_DEFAULT_SOURCE -bt25 "/home/delian/code/v/thirdparty/tcc/lib/libgc.a" -ldl -lpthread "/usr/lib/gcc/x86_64-linux-gnu/6/libatomic.a"
#0 13:41:49 ^ support_flag_when_first_existing ~/code/v>

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