Add alignment support to __libc_allocate_tls().

For statically linked binaries, where all relocation are solved by static
linker, the linker expect that offset to TLS section is aligned. Additionaly,
to maintain absolute alignment, TLS TCB should by also aligned.

Obtained from: CheriBSD (initial version)
MFC after: 1 month
Reviewed by: brooks (previous version), kib
Differential Revision: https://reviews.freebsd.org/D12907