build_support: add g++ cross-compilation wrappers for musl targets

[benhillis]

The existing cross-compilation setup only had C compiler wrappers
(*-underhill-musl-gcc). When cc-rs needed to compile .cxx files, it fell
back to using that gcc wrapper as the C++ compiler, which failed because:

1. gcc doesn't search C++ standard library headers (<cassert>, <cstdint>, etc.)
2. The musl-gcc.specs file uses -nostdinc, stripping all system include
   paths — so even g++ can't find its C++ headers unless they're explicitly
   added back via -isystem

This PR adds matching g++ wrappers for both x86_64 and aarch64 that invoke g++
with the same musl specs, plus -isystem flags pointing at the sysroot's C++
headers. The C++ header version directory is auto-detected via shell glob rather
than hardcoded.

Changes

• build_support/underhill_cross/{x86_64,aarch64}-underhill-musl-g++ — new g++
  wrappers mirroring the existing gcc wrapper pattern
• .cargo/config.toml — added CXX_* env vars for all 4 musl targets so
  cc-rs discovers the wrappers
• shell.nix — added g++ symlinks alongside the existing gcc symlinks

[Copilot]

Pull request overview

Adds C++-aware cross-compilation support for Underhill musl targets by introducing g++ wrapper scripts and wiring them into the build environment so cc-rs can compile .cxx sources against the musl sysroot.

Changes:

• Add *-underhill-musl-g++ wrapper scripts that apply musl-gcc.specs and inject sysroot libstdc++ header include paths.
• Configure Cargo environment variables (CXX_*) so cc-rs discovers the new wrappers for all Underhill musl targets.
• Extend the Nix dev shell to provide *-linux-gnu-g++ symlinks alongside existing gcc symlinks for consistent wrapper resolution.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
shell.nix	Adds g++ symlinks into the temporary wrapper dir so the new scripts can find prefixed compilers in nix shells.
build_support/underhill_cross/x86_64-underhill-musl-g++	New x86_64 musl g++ wrapper with sysroot C++ header auto-detection.
build_support/underhill_cross/aarch64-underhill-musl-g++	New aarch64 musl g++ wrapper with sysroot C++ header auto-detection.
.cargo/config.toml	Adds CXX_* env entries for musl targets so cc-rs uses the new wrappers.

[smalis-msft]

What C++ code are we compiling? Ideally we'd be reducing our non-Rust code, not adding more.