arm64: alternatives: mark patch_alternative() as `noinstr`

The alternatives code must be `noinstr` such that it does not patch itself,
as the cache invalidation is only performed after all the alternatives have
been applied.

Mark patch_alternative() as `noinstr`. Mark branch_insn_requires_update()
and get_alt_insn() with `__always_inline` since they are both only called
through patch_alternative().

Booting a kernel in QEMU TCG with KCSAN=y and ARM64_USE_LSE_ATOMICS=y caused
a boot hang:
[    0.241121] CPU: All CPU(s) started at EL2

The alternatives code was patching the atomics in __tsan_read4() from LL/SC
atomics to LSE atomics.

The following fragment is using LL/SC atomics in the .text section:
  | <__tsan_unaligned_read4+304>:     ldxr    x6, [x2]
  | <__tsan_unaligned_read4+308>:     add     x6, x6, x5
  | <__tsan_unaligned_read4+312>:     stxr    w7, x6, [x2]
  | <__tsan_unaligned_read4+316>:     cbnz    w7, <__tsan_unaligned_read4+304>

This LL/SC atomic sequence was to be replaced with LSE atomics. However since
the alternatives code was instrumentable, __tsan_read4() was being called after
only the first instruction was replaced, which led to the following code in memory:
  | <__tsan_unaligned_read4+304>:     ldadd   x5, x6, [x2]
  | <__tsan_unaligned_read4+308>:     add     x6, x6, x5
  | <__tsan_unaligned_read4+312>:     stxr    w7, x6, [x2]
  | <__tsan_unaligned_read4+316>:     cbnz    w7, <__tsan_unaligned_read4+304>

This caused an infinite loop as the `stxr` instruction never completed successfully,
so `w7` was always 0.

Signed-off-by: Joey Gouly <joey.gouly@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20220405104733.11476-1-joey.gouly@arm.com
Signed-off-by: Will Deacon <will@kernel.org>

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