arm64: Turn "broken gas inst" into real config option

Use the new 'as-instr' Kconfig macro to define CONFIG_BROKEN_GAS_INST
directly, making it available everywhere.

Signed-off-by: Vladimir Murzin <vladimir.murzin@arm.com>
[will: Drop redundant 'y if' logic]
Signed-off-by: Will Deacon <will@kernel.org>