powerpc: Make PT_THREAD_POINTER available to assembly code

PT_THREAD_POINTER is currenty defined inside a #ifndef __ASSEMBLER__ block, but
its usage should not be limited to C code, as it can be useful when accessing
the TLS from assembly code as well.

Reviewed-by: Tulio Magno Quites Machado Filho <tuliom@linux.ibm.com>