libstdc++: Mark pieces of gnu-linux/os_support.h linux-specific

This is notably needed because in glibc 2.34, the move of pthread functions
into libc.so happened for Linux only, not GNU/Hurd.

The pthread_self() function can also always be used fine as it is on
GNU/Hurd.

libstdc++-v3/ChangeLog:

	* config/os/gnu-linux/os_defines.h [!__linux__]
	(_GLIBCXX_NATIVE_THREAD_ID, _GLIBCXX_GTHREAD_USE_WEAK): Do not define.

Reviewed-by: Jonathan Wakely <jwakely@redhat.com>