ARM: 9122/1: select HAVE_FUTEX_CMPXCHG

tglx notes:
  This function [futex_detect_cmpxchg] is only needed when an
  architecture has to runtime discover whether the CPU supports it or
  not.  ARM has unconditional support for this, so the obvious thing to
  do is the below.

Fixes linkage failure from Clang randconfigs:
kernel/futex.o:(.text.fixup+0x5c): relocation truncated to fit: R_ARM_JUMP24 against `.init.text'
and boot failures for CONFIG_THUMB2_KERNEL.

Link: ClangBuiltLinux/linux#325

Comments from Nick Desaulniers:

 See-also: 03b8c7b ("futex: Allow architectures to skip
 futex_atomic_cmpxchg_inatomic() test")

Reported-by: Arnd Bergmann <[email protected]>
Reported-by: Nathan Chancellor <[email protected]>
Suggested-by: Thomas Gleixner <[email protected]>
Signed-off-by: Nick Desaulniers <[email protected]>
Reviewed-by: Thomas Gleixner <[email protected]>
Tested-by: Nathan Chancellor <[email protected]>
Reviewed-by: Linus Walleij <[email protected]>
Cc: [email protected] # v3.14+
Reviewed-by: Arnd Bergmann <[email protected]>
Signed-off-by: Russell King (Oracle) <[email protected]>

Author: nickdesaulniers

arch/arm/Kconfig

@@ -92,6 +92,7 @@ config ARM
 	select HAVE_FTRACE_MCOUNT_RECORD if !XIP_KERNEL
 	select HAVE_FUNCTION_GRAPH_TRACER if !THUMB2_KERNEL && !CC_IS_CLANG
 	select HAVE_FUNCTION_TRACER if !XIP_KERNEL
+	select HAVE_FUTEX_CMPXCHG if FUTEX
 	select HAVE_GCC_PLUGINS
 	select HAVE_HW_BREAKPOINT if PERF_EVENTS && (CPU_V6 || CPU_V6K || CPU_V7)
 	select HAVE_IRQ_TIME_ACCOUNTING