powerpc64/ftrace: Nop out additional 'std' instruction emitted by gcc v5.x

Gcc v5.x emits a 3-instruction sequence for -mprofile-kernel:
	mflr	r0
	std	r0, 16(r1)
	bl	_mcount

Gcc v6.x moved to a simpler 2-instruction sequence by removing the 'std'
instruction. The store saved the return address in the LR save area in
the caller stack frame for stack unwinding. However, with dynamic
ftrace, we no longer have a call to _mcount on kernel boot when ftrace
is not enabled. When ftrace is enabled, that store is performed within
ftrace_caller(). As such, the additional 'std' instruction is redundant.
Nop it out on kernel boot.

With this change, we now use the same 2-instruction profiling sequence
with both -mprofile-kernel, as well as -fpatchable-function-entry on
64-bit powerpc.

Signed-off-by: Naveen N Rao <naveen@kernel.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://patch.msgid.link/20241030070850.1361304-4-hbathini@linux.ibm.com