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: https://github.com/ClangBuiltLinux/linux/issues/325

Comments from Nick Desaulniers:

 See-also: 03b8c7b6 ("futex: Allow architectures to skip
 futex_atomic_cmpxchg_inatomic() test")
Reported-by: Arnd Bergmann <arnd@arndb.de>
Reported-by: Nathan Chancellor <nathan@kernel.org>
Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Cc: stable@vger.kernel.org # v3.14+
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>