Fix ppc_collect/supply_ptrace_register() routines

Fix ppc_collect/supply_ptrace_register() routines

This patch fixes the routines to collect and supply ptrace registers on ppc64le
gdbserver. Originally written for big endian arch, they were causing several
issues on little endian. With this fix, the number of unexpected failures in
the testsuite dropped from 263 to 72 on ppc64le.

gdb/gdbserver/ChangeLog

	* linux-ppc-low.c (ppc_collect_ptrace_register): Adjust routine to take
	endianness into account.
	(ppc_supply_ptrace_register): Likewise.