kbuild: Add support for 'as-instr' to be used in Kconfig files

Similar to 'cc-option' or 'ld-option', it is occasionally necessary to
check whether the assembler supports certain ISA extensions. In the
arm64 code we currently do this in Makefile with an additional define:

lseinstr := $(call as-instr,.arch_extension lse,-DCONFIG_AS_LSE=1)

Add the 'as-instr' option so that it can be used in Kconfig directly:

	def_bool $(as-instr,.arch_extension lse)

Acked-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Vladimir Murzin <vladimir.murzin@arm.com>
Tested-by: Vladimir Murzin <vladimir.murzin@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>