ARM: 9144/1: forbid ftrace with clang and thumb2_kernel

ARM: 9144/1: forbid ftrace with clang and thumb2_kernel

clang fails to build kernels with THUMB2 and FUNCTION_TRACER
enabled when there is any inline asm statement containing
the frame pointer register r7:

arch/arm/mach-exynos/mcpm-exynos.c:154:2: error: inline asm clobber list contains reserved registers: R7 [-Werror,-Winline-asm]
arch/arm/probes/kprobes/actions-thumb.c:449:3: error: inline asm clobber list contains reserved registers: R7 [-Werror,-Winline-asm]

Apparently gcc should also have warned about this, and the
configuration is actually invalid, though there is some
disagreement on the bug trackers about this.

Link: https://bugs.llvm.org/show_bug.cgi?id=45826
Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94986
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>