x86: Add prefix symbols for function padding

When code is compiled with:

  -fpatchable-function-entry=${PADDING_BYTES},${PADDING_BYTES}

functions will have PADDING_BYTES of NOP in front of them. Unwinders
and other things that symbolize code locations will typically
attribute these bytes to the preceding function.

Given that these bytes nominally belong to the following symbol this
mis-attribution is confusing.

Inspired by the fact that CFI_CLANG emits __cfi_##name symbols to
claim these bytes, use objtool to emit __pfx_##name symbols to do
the same when CFI_CLANG is not used.

This then shows the callthunk for symbol 'name' as:

  __pfx_##name+0x6/0x10
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Yujie Liu <yujie.liu@intel.com>
Link: https://lkml.kernel.org/r/20221028194453.592512209@infradead.org

arch/x86/Kconfig

@@ -2471,6 +2471,10 @@ config CALL_THUNKS
 	def_bool n
 	select FUNCTION_ALIGNMENT_16B
 
+config PREFIX_SYMBOLS
+	def_bool y
+	depends on CALL_THUNKS && !CFI_CLANG
+
 menuconfig SPECULATION_MITIGATIONS
 	bool "Mitigations for speculative execution vulnerabilities"
 	default y

scripts/Makefile.lib

@@ -265,6 +265,7 @@ objtool-args-$(CONFIG_STACK_VALIDATION)			+= --stackval
 objtool-args-$(CONFIG_HAVE_STATIC_CALL_INLINE)		+= --static-call
 objtool-args-$(CONFIG_HAVE_UACCESS_VALIDATION)		+= --uaccess
 objtool-args-$(CONFIG_GCOV_KERNEL)			+= --no-unreachable
+objtool-args-$(CONFIG_PREFIX_SYMBOLS)			+= --prefix=$(CONFIG_FUNCTION_PADDING_BYTES)
 
 objtool-args = $(objtool-args-y)					\
 	$(if $(delay-objtool), --link)					\