feat(wasm): link libsetjmp, so setjmp needs no flag

-mllvm -wasm-enable-sjlj, in every wasm compile because setjmp.h does
not compile without it, turns setjmp and longjmp into calls to
__wasm_setjmp, __wasm_setjmp_test and __wasm_longjmp and a throw of
the __c_longjmp tag, all of which live in wasi-libc's libsetjmp. The
driver does not link it, so a program that used setjmp, or a C library
that handles its errors that way, failed to link with undefined
__wasm_setjmp until the project put -lsetjmp in its linkFlags.

Build() adds it to every wasm link, next to -lunwind. A static archive
contributes only the members something references, so a program
without setjmp gets nothing from it, and every sysroot that passes the
exception check ships it. WasmExceptions drops its own -lsetjmp, and
its C fixture now fails to link without this.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/WasmExceptions/main.cpp

@@ -12,7 +12,8 @@ using namespace Crafter;
 // libc++.a itself (stoi), and with an argument lets one escape main, which
 // has to end the run as an uncaught exception rather than exit 0, trap or
 // hang. Its C source uses setjmp, which compiles and links next to the C++
-// objects only when C gets the same wasm flags.
+// objects only when C gets the same wasm flags, and links at all only
+// because the build adds libsetjmp: the configuration asks for nothing.
 //
 // wasm builds used to be compiled with -fno-exceptions, so a throw did not
 // compile and libc++'s own throws aborted. Each part of turning them on fails
@@ -55,9 +56,6 @@ int main() {
     std::array<fs::path, 1> impls = { "main" };
     cfg.GetInterfacesAndImplementations(ifaces, impls);
     cfg.cFiles = { cfg.path / "jump" };
-    // setjmp and longjmp live in libsetjmp, which the build does not link
-    // on its own.
-    cfg.linkFlags.push_back("-lsetjmp");
 
     std::unordered_map<fs::path, std::shared_future<BuildResult>> depResults;
     std::mutex depMutex;