nptl: Increase default TCB alignment to 32

rseq support will use a 32-byte aligned field in struct pthread,
so the whole struct needs to have at least that alignment.

nptl/tst-tls3mod.c uses TCB_ALIGNMENT, therefore include <descr.h>
to obtain the fallback definition.

Reviewed-by: H.J. Lu <hjl.tools@gmail.com>