powerpc: enable initial-exec TLS

[pkubaj]

Summary

Use initial-exec, like other architectures.

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

[emaste]

lib/libc/Makefile

35

Is this just for consistency?

I should add a comment explaining this conditional - I wondered where this came from, and was surprised to find I added it in 0f61170882cb39a3fa4b76bcfc9c8acd2404a2da :)

I will do that after your commit though.

[pkubaj]

lib/libc/Makefile

35

Yes, since it's only for consistency and doesn't actually change anything.