powerpc: enable initial-exec TLS

Summary:
Use initial-exec, like other architectures.

While here, switch MACHINE_ARCH in lib/libc/Makefile to LIBC_ARCH and consistently use powerpc.

Subscribers: imp, Contributor Reviews (src)

Differential Revision: https://reviews.freebsd.org/D34315
Reviewed by: luporl
MFC after: 2 weeks