arm64: assembler: remove user_alt

arm64: assembler: remove user_alt

user_alt isn't being used anymore. It's also simpler and clearer
to directly use alternative_insn and _cond_extable in-line when
needed.

Reported-by: Mark Rutland <mark.rutland@arm.com>
Link: https://lore.kernel.org/linux-arm-kernel/20210520125735.GF17233@C02TD0UTHF1T.local/
Signed-off-by: Fuad Tabba <tabba@google.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20210524083001.2586635-8-tabba@google.com
Signed-off-by: Will Deacon <will@kernel.org>