arm64: unify asm-arch manipulation

arm64: unify asm-arch manipulation

Assemblers will reject instructions not supported by a target
architecture version, and so we must explicitly tell the assembler the
latest architecture version for which we want to assemble instructions
from.

We've added a few AS_HAS_ARMV8_<N> definitions for this, in addition to
an inconsistently named AS_HAS_PAC definition, from which arm64's
top-level Makefile determines the architecture version that we intend to
target, and generates the `asm-arch` variable.

To make this a bit clearer and easier to maintain, this patch reworks
the Makefile to determine asm-arch in a single if-else-endif chain.
AS_HAS_PAC, which is defined when the assembler supports
`-march=armv8.3-a`, is renamed to AS_HAS_ARMV8_3.

As the logic for armv8.3-a is lifted out of the block handling pointer
authentication, `asm-arch` may now be set to armv8.3-a regardless of
whether support for pointer authentication is selected. This means that
it will be possible to assemble armv8.3-a instructions even if we didn't
intend to, but this is consistent with our handling of other
architecture versions, and the compiler won't generate armv8.3-a
instructions regardless.

For the moment there's no need for an CONFIG_AS_HAS_ARMV8_1, as the code
for LSE atomics and LDAPR use individual `.arch_extension` entries and
do not require the baseline asm arch to be bumped to armv8.1-a. The
other armv8.1-a features (e.g. PAN) do not require assembler support.

There should be no functional change as a result of this patch.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Mark Brown <broonie@kernel.org>
Cc: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20230131105809.991288-2-mark.rutland@arm.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>