powerpc/ftrace: Disable ftrace on ppc32 if using clang

Ftrace on ppc32 expects a three instruction sequence at the beginning of
each function when specifying -pg:
	mflr	r0
	stw	r0,4(r1)
	bl	_mcount

This is the case with all supported versions of gcc. Clang however emits
a branch to _mcount after the function prologue, similar to the pre
-mprofile-kernel ABI on ppc64. This is not supported.

Disable ftrace on ppc32 if using clang for now. This can be re-enabled
later if clang picks up support for -fpatchable-function-entry on ppc32.

Signed-off-by: Naveen N Rao <naveen@kernel.org>
Acked-by: Nick Desaulniers <ndesaulniers@google.com>
Link: https://github.com/llvm/llvm-project/issues/63220
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230609034501.407971-1-naveen@kernel.org