BACKPORT: ARM: 8905/1: Emit __gnu_mcount_nc when using Clang 10.0.0 o…

…r newer

commit b0fe66cf095016e0b238374c10ae366e1f087d11 upstream.

Currently, multi_v7_defconfig + CONFIG_FUNCTION_TRACER fails to build
with clang:

arm-linux-gnueabi-ld: kernel/softirq.o: in function `_local_bh_enable':
softirq.c:(.text+0x504): undefined reference to `mcount'
arm-linux-gnueabi-ld: kernel/softirq.o: in function `__local_bh_enable_ip':
softirq.c:(.text+0x58c): undefined reference to `mcount'
arm-linux-gnueabi-ld: kernel/softirq.o: in function `do_softirq':
softirq.c:(.text+0x6c8): undefined reference to `mcount'
arm-linux-gnueabi-ld: kernel/softirq.o: in function `irq_enter':
softirq.c:(.text+0x75c): undefined reference to `mcount'
arm-linux-gnueabi-ld: kernel/softirq.o: in function `irq_exit':
softirq.c:(.text+0x840): undefined reference to `mcount'
arm-linux-gnueabi-ld: kernel/softirq.o:softirq.c:(.text+0xa50): more undefined references to `mcount' follow

clang can emit a working mcount symbol, __gnu_mcount_nc, when
'-meabi gnu' is passed to it. Until r369147 in LLVM, this was
broken and caused the kernel not to boot with '-pg' because the
calling convention was not correct. Always build with '-meabi gnu'
when using clang but ensure that '-pg' (which is added with
CONFIG_FUNCTION_TRACER and its prereq CONFIG_HAVE_FUNCTION_TRACER)
cannot be added with it unless this is fixed (which means using
clang 10.0.0 and newer).

Link: ClangBuiltLinux/linux#35
Link: https://bugs.llvm.org/show_bug.cgi?id=33845
Link: llvm/llvm-project@16fa8b0

Reviewed-by: Matthias Kaehlcke <mka@chromium.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>

Bug: 145525910
[nd: Kconfig clang version check removed due to significant size of
     backporting 469cb7376c06/2fd5b09c201e. Android R will not have
     a version of Clang < clang-10]
Change-Id: I186771b7a3553652b84ee66dac6b53f7932b7076
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>

arch/arm/Makefile

@@ -118,6 +118,10 @@ ifeq ($(CONFIG_ARM_UNWIND),y)
 CFLAGS_ABI	+=-funwind-tables
 endif
 
+ifeq ($(cc-name),clang)
+CFLAGS_ABI	+= -meabi gnu
+endif
+
 ifeq ($(CONFIG_THUMB2_KERNEL),y)
 AFLAGS_AUTOIT	:=$(call as-option,-Wa$(comma)-mimplicit-it=always,-Wa$(comma)-mauto-it)
 AFLAGS_NOWARN	:=$(call as-option,-Wa$(comma)-mno-warn-deprecated,-Wa$(comma)-W)