nptl: Add C11 threads tss_* functions

nptl: Add C11 threads tss_* functions

This patch adds the tss_* definitions from C11 threads (ISO/IEC 9899:2011),
more specifically tss_create, tss_delete, tss_get, tss_set, and required
types.

Mostly of the definitions are composed based on POSIX conterparts, including
tss_t (pthread_key_t).

Checked with a build for all major ABI (aarch64-linux-gnu, alpha-linux-gnu,
arm-linux-gnueabi, i386-linux-gnu, ia64-linux-gnu, m68k-linux-gnu,
microblaze-linux-gnu [1], mips{64}-linux-gnu, nios2-linux-gnu,
powerpc{64le}-linux-gnu, s390{x}-linux-gnu, sparc{64}-linux-gnu,
and x86_64-linux-gnu).

Also ran a full check on aarch64-linux-gnu, x86_64-linux-gnu, i686-linux-gnu,
arm-linux-gnueabhf, and powerpc64le-linux-gnu.

	[BZ #14092]
	* conform/data/threads.h-data (thread_local): New macro.
	(TSS_DTOR_ITERATIONS): Likewise.
	(tss_t): New type.
	(tss_dtor_t): Likewise.
	(tss_create): New function.
	(tss_get): Likewise.
	(tss_set): Likewise.
	(tss_delete): Likewise.
	* nptl/Makefile (libpthread-routines): Add tss_create, tss_delete,
	tss_get, and tss_set objects.
	* nptl/Versions (libpthread) [GLIBC_2.28]: Likewise.
	* nptl/tss_create.c: New file.
	* nptl/tss_delete.c: Likewise.
	* nptl/tss_get.c: Likewise.
	* nptl/tss_set.c: Likewise.
	* sysdeps/nptl/threads.h (thread_local): New define.
	(TSS_DTOR_ITERATIONS): Likewise.
	(tss_t): New typedef.
	(tss_dtor_t): Likewise.
	(tss_create): New prototype.
	(tss_get): Likewise.
	(tss_set): Likewise.
	(tss_delete): Likewise.