arm64: Silence spurious kbuild warning on menuconfig

Since bbb56c27 ("arm64: Add detection code for broken .inst support
in binutils"), running any make target that doesn't involve the cross
compiler results in a spurious warning:

$ make ARCH=arm64 menuconfig
arch/arm64/Makefile:43: Detected assembler with broken .inst; disassembly will be unreliable

while

$ make ARCH=arm64 CROSS_COMPILE=aarch64-arm-linux- menuconfig

is silent (assuming your compiler is not affected). That's because
the code that tests for the workaround is always run, irrespective
of the current configuration being available or not.

An easy fix is to make the detection conditional on CONFIG_ARM64
being defined, which is only the case when actually building
something.

Fixes: bbb56c27 ("arm64: Add detection code for broken .inst support in binutils")
Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>