libstdc++: Rename __null_terminated to avoid collision with Apple SDK

libstdc++: Rename __null_terminated to avoid collision with Apple SDK

The macOS 13 SDK (and equivalent-version iOS and other Apple OS SDKs)
contain this definition in <sys/cdefs.h>:

863  #define __null_terminated

This collides with the use of __null_terminated in libstdc++'s
experimental fs_path.h.

As libstdc++'s use of this token is entirely internal to fs_path.h, the
simplest workaround, renaming it, is most appropriate. Here, it's
renamed to __nul_terminated, referencing the NUL ('\0') value that is
used to terminate the strings in the context in which this tag structure
is used.

libstdc++-v3/ChangeLog:

	* include/experimental/bits/fs_path.h (__detail::__null_terminated):
	Rename to __nul_terminated to avoid colliding with a macro in
	Apple's SDK.

Signed-off-by: Mark Mentovai <mark@mentovai.com>