Fix TSD stubs for non-initial-exec case (fixes #5667).

Fix TSD stubs for non-initial-exec case (fixes #5667).

ppc64le TSD disabled for now since I am insufficiently familiar with ppc
asm. x86 pthread stubs deleted because adding USE_ELF_TLS to the #if is
isn't worth the effort since it only saves a single function call on
initial entry (TSD stubs are not used for read-only text now). also
potentially fix non-pthread TSD builds (_glapi_Dispatch was undefined),
but untested (could still be broken).

Tested-by: Jesse Natalie <jenatali@microsoft.com>
Tested-by: Jan Beich <jbeich@freebsd.org>
Signed-off-by: Alex Xu (Hello71) <alex_y_xu@yahoo.ca>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/13935>