powerpc: Enforce compiler barriers on hardware transactions

Work around a GCC behavior with hardware transactional memory built-ins.
GCC doesn't treat the PowerPC transactional built-ins as compiler
barriers, moving instructions past the transaction boundaries and
altering their atomicity.

(cherry picked from commit 42bf1c89)

Conflicts:
	sysdeps/unix/sysv/linux/powerpc/elision-trylock.c