Add LLVM 23 build support

[dgryski]

Adds llvm23 build-tag config, mirroring the existing llvm22 support.
Requires tinygo-org/go-llvm#79, which is not yet merged/tagged, so go.mod
is left pointing at the current go-llvm commit for now.

Verified against real LLVM 23.1.1 headers and libraries (via Homebrew): both
a native build and a wasi build produce working binaries that run correctly.

LLVM 23 keeps the wasm32-unknown-wasi triple deprecation and the Xtensa
feature renames introduced in LLVM 22 (confirmed for the triple; the Xtensa
renames are assumed unchanged, not yet verified against an Xtensa-enabled
LLVM 23 toolchain).

LLVM 23 removed the Os/Oz pipeline levels from the new pass manager's
textual pipeline syntax ("thinlto-pre-link<Os>" now aborts with a fatal
LLVM error). transform/optimizer.go now runs the O2 pipeline level
instead on LLVM 23+, relying on the optsize/minsize function attributes
already set elsewhere to keep steering size-conscious passes.

LLVM 23 requires the llvm.returnaddress intrinsic's mangled name to include
its pointer type suffix (llvm.returnaddress.p0). Patches the linkName for
that specific //export pragma in compiler/symbol.go.

LLVM 23 splits the conditional/unconditional branch instruction into separate
opcodes and, more importantly, reorders a conditional branch's operands from
(cond, else, then) to (cond, then, else). interp/compiler.go hardcoded
the old order when building its own branch instruction, which silently
inverted every branch condition evaluated by the compile-time interpreter
under LLVM 23, sending essentially all packages into infinite recursion
during interp. Fixed with a version-gated operand read.

Also normalizes away the LLVM 23 nosync attribute (newly inferred for
llvm.memcpy/llvm.memmove) in the golden-IR test comparators in
compiler_test.go, transform_test.go and interp_test.go, and updates a
hardcoded "Oz" pipeline string in transform/unwind_test.go the same way
as the production code path.

Known gaps

• A handful of golden-IR-text comparison tests in compiler_test.go show
  cosmetic diffs from LLVM 23's own output-formatting changes (float
  hex-constant syntax, a GEP-canonicalization preference change, and
  attribute-group renumbering). These are optimizer/printer text drift, not
  correctness bugs; fixing them properly needs per-LLVM-version golden
  fixtures, which is out of scope here.
• The Xtensa feature-rename compatibility shim (compileopts/features_llvm22.go)
  is extended to LLVM 23 by assumption, not verified against a real
  Xtensa-enabled LLVM 23 toolchain.

🤖 Generated with Claude Code

[jakebailey]

Out of order here (why are some files one way, some the other)

[dgryski]

This passes my test corpus as well as the stdlib tests on darwin/arm64 and wasip1.

[dgryski]

Will need to merge tinygo-org/go-llvm#79 first for llvm23 support and then update go.mod.