## Summary of changes

Attempt to fix macOS native build failure caused by the macos-14 runner
image updating llvm@15 to x86_64-only.

## Reason for change

CI is broken

The macos-14 runner image (20260316.0303) rebuilt Homebrew's llvm@15 as
x86_64-only. This breaks our arm64 cross-compilation because Homebrew's
libunwind.dylib shadows the system's universal version in the linker
search path. The linker finds the x86_64-only copy, ignores it for
arm64, and fails to resolve __Unwind_Resume.

## Implementation details

From Claude:

 The fix has two layers:

**1. Build only the static re2 library on macOS**
(`build/cmake/FindRe2.cmake`)

Changed `$(MAKE) -j` to `$(MAKE) -j obj/libre2.a` for both arm64 and
x86_64 re2 builds. The default make target builds both `libre2.a`
(static) and `libre2.dylib` (shared). The shared library link fails
because the linker picks up the x86_64-only libunwind. We only use the
static library (`IMPORTED_LOCATION` and `BUILD_BYPRODUCTS` both
reference `obj/libre2.a`), so the shared library was never needed.

**2. Isolate macOS builds from Homebrew via SDK sysroot**
(`Build.Steps.cs`, `Build.Shared.Steps.cs`)

- Resolve the macOS SDK path via `xcrun --show-sdk-path` and pass it as
`-DCMAKE_OSX_SYSROOT` to cmake. This causes cmake to pass
`-Wl,-syslibroot,<path>` to the linker, which prepends the sysroot to
all
default search directories. `/usr/local/lib` becomes
`<sysroot>/usr/local/lib` (doesn't exist in the SDK, so Homebrew's
libunwind is never found) while `/usr/lib` becomes `<sysroot>/usr/lib`
(contains TBD
  stubs for the real universal system libunwind).
- Clear `LDFLAGS` and `LIBRARY_PATH` environment variables to prevent
any additional Homebrew-injected paths from leaking into the build.

Applied to both `CompileNativeSrcMacOs` (tracer) and
`CompileNativeLoaderOsx` (native loader).


## Test coverage

Ran a build and it did work, now to see if this _actually_ works then
maybe we are good 🤷

## Other details
FWIW I don't really have a great understanding of the macOS build
Re-attempt of #8347

Generated with Claude