fix: statically link build.mcpp host helpers with musl toolchains

[wellwei]

Reproduction

On a fresh Ubuntu 24.04 ARM64 host, mcpp selects gcc@15.1.0-musl as the native toolchain. A project containing even this minimal build program fails:

#include <cstdio>
int main() {
    std::puts("build helper ran");
    return 0;
}

build.mcpp compiling
build.mcpp running
build.mcpp exited with 127

readelf shows target/.build-mcpp/build.mcpp.bin is dynamically linked with interpreter /lib/ld-musl-aarch64.so.1. The glibc Ubuntu host does not provide that loader, and strace reports execve(.../build.mcpp.bin, ...) = -1 ENOENT.

Root cause

prepare.cppm correctly defaults musl project output to static linkage, but build_program.cppm constructs the host-helper link command independently and does not add -static. process.cppm then collapses the posix_spawnp error into exit 127 with empty output.

Expected behavior

• A build program compiled with a musl host toolchain is fully static and runs without a host musl loader.
• A posix_spawnp failure retains a useful error message instead of returning an unexplained empty exit 127.

Proposed fix

• Add -static only to the final build.mcpp helper link when the resolved host toolchain targets musl.
• Rev the build-program cache identity so older helper-link policy caches are invalidated.
• Preserve the direct posix_spawnp error code and message in captured output.
• Add native Ubuntu ARM64 fresh-install coverage that runs a minimal build.mcpp and asserts the helper has no PT_INTERP.

Environment

• mcpp: 0.0.109
• Host: Ubuntu 24.04 ARM64 (glibc)
• Toolchain selected by mcpp: gcc@15.1.0-musl