powerpc: Fix build issue with LD_DEAD_CODE_DATA_ELIMINATION and...

powerpc: Fix build issue with LD_DEAD_CODE_DATA_ELIMINATION and FTRACE_MCOUNT_USE_PATCHABLE_FUNCTION_ENTRY

We recently added support for -fpatchable-function-entry and it is
enabled by default on ppc32 (ppc64 needs gcc v13.1.0). When building the
kernel for ppc32 and also enabling CONFIG_LD_DEAD_CODE_DATA_ELIMINATION,
we see the below build error with older gcc versions:
  powerpc-linux-gnu-ld: init/main.o(__patchable_function_entries): error: need linked-to section for --gc-sections

This error is thrown since __patchable_function_entries section would be
garbage collected with --gc-sections since it does not reference any
other kept sections. This has subsequently been fixed with:
  https://sourceware.org/git/?p=binutils-gdb.git;a=commitdiff;h=b7d072167715829eed0622616f6ae0182900de3e

Disable LD_DEAD_CODE_DATA_ELIMINATION for gcc versions before v11.1.0 if
using -fpatchable-function-entry to avoid this bug.

Fixes: 0f71dcfb ("powerpc/ftrace: Add support for -fpatchable-function-entry")
Reported-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Naveen N Rao <naveen@kernel.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230913134129.2782088-1-naveen@kernel.org

arch/powerpc/Kconfig

@@ -255,7 +255,7 @@ config PPC
 	select HAVE_KPROBES
 	select HAVE_KPROBES_ON_FTRACE
 	select HAVE_KRETPROBES
-	select HAVE_LD_DEAD_CODE_DATA_ELIMINATION if HAVE_OBJTOOL_MCOUNT
+	select HAVE_LD_DEAD_CODE_DATA_ELIMINATION if HAVE_OBJTOOL_MCOUNT && (!ARCH_USING_PATCHABLE_FUNCTION_ENTRY || (!CC_IS_GCC || GCC_VERSION >= 110100))
 	select HAVE_LIVEPATCH			if HAVE_DYNAMIC_FTRACE_WITH_REGS
 	select HAVE_MOD_ARCH_SPECIFIC
 	select HAVE_NMI				if PERF_EVENTS || (PPC64 && PPC_BOOK3S)