Add a missing valid triple "aarch64-apple-macosx" for usability.

The .syntax unified directive and .codeX/.code X directives are, other
than some simple common printing code, exclusively implemented in the
targets themselves. Thus, remove the corresponding MCAF_* flags and
reimplement the directives solely within the targets. This avoids
exposing all targets to all other targets' flags.

Since MCAF_SubsectionsViaSymbols is all that remains, convert it to its
own function like other directives, simplifying its implementation.

Note that, on X86, we now always need a target streamer when parsing
assembly, as it's now used for directives that aren't COFF-specific. It
still does not however need to do anything when producing a non-COFF
object file, so this commit does not introduce any new target streamers.

There is some churn in test output, and corresponding UTC regex changes,
due to comments no longer being flushed by these various directives (and
EmitEOL is not exposed outside MCAsmStreamer.cpp so we couldn't do so
even if we wanted to), but that was a bit odd to be doing anyway.

This is motivated by Morello LLVM, which adds yet another assembler flag
to distinguish A64 and C64 instruction sets, but did not update every
switch and so emits warnings during the build. Rather than fix those
warnings it seems better to instead make the problem not exist in the
first place via this change.

Regenerate the failing test as well.

Updating `llvm/test/CodeGen/AMDGPU/GlobalISel/extractelement.ll` with
`update_llc_test_checks.py` ended with several kernels with no checks.

Llc's output contained the line ".amdgpu_hsa_kernel <funcname>" after
the ".type <funcname>,@function" entry which was not considered by the
regexp.

Having the version in `get_run_handlers` meant that a RUN line without the
version in it would not match, and therefore UTC would generate a useless
catch-all check of the form:

```
;; NOTE: These prefixes are unused and the list is autogenerated. Do not add tests below this line:
; CHECK: {{.*}}
```

This patch also adds `arm64_32` as a base-level run handler, and assumes it
will always apply to a darwin target (which is currently the case for all
arm64_32-* triples I see in tree currently).

A full downstream fork can already hack up update_llc_test_checks.py to
support custom tools that output assembly.

An out-of-tree frontend which is meant to build against upstream
llvm-project cannot do this, and so providing additional arguments to
support a non-standard tool is useful.

This also makes a minor adjustment to the regular expression for
matching AMDGPU functions when fewer comments are enabled, which happens
to be the case for our out-of-tree shader compiler (which motivated this
change).

Labels are matched using a regexp of the form '^(pattern):', which
requires the addition of a "suffix" concept to NamelessValue.

Aside from that, the key challenge is that block labels are values, and
we typically capture values including the prefix '%'. However, when
labels appear at the start of a basic block, the prefix '%' is not
included, so we must capture block label values *without* the prefix
'%'.

We don't know ahead of time whether an IR value is a label or not. In
most cases, they are prefixed by the word "label" (their type), but this
isn't the case in phi nodes. We solve this issue by leveraging the
two-phase nature of variable generalization: the first pass finds all
occurences of a variable and determines whether the '%' prefix can be
included or not. The second pass does the actual substitution.

This change also unifies the generalization path for assembly with that
for IR and analysis, in the hope that any future changes avoid diverging
those cases future.

I also considered the alternative of trying to detect the phi node case
using more regular expression special cases but ultimately decided
against that because it seemed more fragile, and perhaps the approach of
keeping a tentative prefix that may later be discarded could also be
eventually applied to some metadata and attribute cases.

Note that an early version of this change was reviewed as
https://reviews.llvm.org/D142452, before version numbers were
introduced. This is a substantially updated version of that change.

arm64-apple-macosx is the default triple (usually with the macOS version
number) on arm64 macOS. Support it in update_llc_test_checks.py.

Support for SPIRV added, updated test SPV_INTEL_optnone.ll using the script.

Previously https://reviews.llvm.org/D157858