powerpc: Undefine Linux ptrace macros that conflict with __ptrace_request

Linux ptrace headers define macros whose tokens conflict with the
constants of enum __ptrace_request causing build errors when
asm/ptrace.h or linux/ptrace.h are included before sys/ptrace.h.

	* sysdeps/unix/sysv/linux/powerpc/sys/ptrace.h: Undefine Linux
	macros used in __ptrace_request.

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