musl host support: bootstrap and run on Alpine (x86_64-alpine-linux-musl)

The Linux host code paths were gated on the x86_64_pc_linux_gnu target
define alone, and LoadProject hardcoded --target=x86_64-pc-linux-gnu when
compiling project.cpp and the module PCMs. Both break on musl hosts: the
glibc release launcher cannot run there at all (no glibc loader; gcompat
lacks libgcc_s and the __isoc23_* symbols), and a musl-built launcher
would still compile project.cpp for the wrong libc.

- The host gates test clang's __linux__ instead of one triple's define:
  they mean "Linux host", whatever the libc (or, later, the CPU).
- LoadProject derives the host triple from HostTarget() (clang's
  -print-target-triple), which is also more correct on glibc distros
  whose triple isn't x86_64-pc-linux-gnu.
- project.cpp / the cmake libc++ flags treat any *-linux-gnu or
  *-linux-musl target as Linux.
- build.sh targets the host toolchain's triple (CRAFTER_BUILD_TARGET
  overrides) and derives the per-target define from it. On Arch this is
  byte-for-byte the previous behaviour.
- CI: a release-musl job bootstraps on alpine:edge, runs the tests, and
  attaches crafter-build-linux-x86_64-musl-v2.tar.gz to the rolling
  latest release (via the API, so the glibc assets survive). Dynamic
  against musl libc++: the launcher dlopens project.so, so it cannot be
  fully static.

First consumer: imsd's package CI, which cross-compiles for aarch64 from
an Alpine container.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

implementations/Crafter.Build-Clang.cpp

@@ -307,7 +307,7 @@ CompileCommand Crafter::GetCompileCommand(const Configuration& config) {
     }
 
     if(config.type == ConfigurationType::LibraryDynamic) {
-        #ifdef CRAFTER_BUILD_CONFIGURATION_TARGET_x86_64_pc_linux_gnu
+        #ifdef __linux__
         out.command += " -fPIC -D CRAFTER_BUILD_CONFIGURATION_TYPE_SHARED_LIBRARY";
         #endif
         #if defined(CRAFTER_BUILD_CONFIGURATION_TARGET_x86_64_pc_windows_msvc) || defined(CRAFTER_BUILD_CONFIGURATION_TARGET_x86_64_w64_mingw32)
@@ -453,7 +453,7 @@ BuildResult Crafter::Build(Configuration& config, std::unordered_map<fs::path, s
     // users supply cfg.sysroot pointing at their wasi-sdk install. Covers all
     // wasm32-* triples (wasi, wasip1, wasip2, ...); the sysroot's per-triple
     // subdirs handle the differences.
-    #ifdef CRAFTER_BUILD_CONFIGURATION_TARGET_x86_64_pc_linux_gnu
+    #ifdef __linux__
     if (config.sysroot.empty() && config.target.starts_with("wasm32")) {
         config.sysroot = "/usr/share/wasi-sysroot";
     }
@@ -1131,7 +1131,7 @@ BuildResult Crafter::Build(Configuration& config, std::unordered_map<fs::path, s
     if(buildResult.repack) {
         Progress::Task task(std::format("Linking {}", config.outputName));
         if(config.type == ConfigurationType::Executable) {
-            #ifdef CRAFTER_BUILD_CONFIGURATION_TARGET_x86_64_pc_linux_gnu
+            #ifdef __linux__
             if(config.target == "x86_64-w64-mingw32") {
                 try {
                     // Copy any LibraryDynamic dependency DLLs alongside
@@ -1204,7 +1204,7 @@ BuildResult Crafter::Build(Configuration& config, std::unordered_map<fs::path, s
             }
             #endif
         } else if(config.type == ConfigurationType::LibraryStatic) {
-            #ifdef CRAFTER_BUILD_CONFIGURATION_TARGET_x86_64_pc_linux_gnu
+            #ifdef __linux__
             // ThinLTO emits LLVM bitcode objects; plain `ar` writes an archive
             // index that omits their symbols, so a consumer's lld link can't
             // pull the needed members. llvm-ar writes a bitcode-aware index.
@@ -1550,7 +1550,7 @@ ArgQuery Crafter::ApplyStandardArgs(Configuration& cfg, std::span<const std::str
     // includes it. (Build() also runs this once more for callers that bypassed
     // ApplyStandardArgs, but doing it here makes dep PcmDirs consistent
     // between the consumer's command-construction and the dep's own build.)
-    #ifdef CRAFTER_BUILD_CONFIGURATION_TARGET_x86_64_pc_linux_gnu
+    #ifdef __linux__
     if (cfg.sysroot.empty() && cfg.target.starts_with("wasm32")) {
         cfg.sysroot = "/usr/share/wasi-sysroot";
     }