fix(wasm): what the review found

-flto in a wasm configuration's compileFlags or linkFlags reached the
link, where LTO code generation runs without the backend options that
keep exception handling: every landing pad was dropped, and the build
succeeded with throws that escaped their catch blocks. -fno-exceptions
there made every `import std;` fail on the std module built with
exceptions, and a wasm shared library failed to link against the eh/
libc++, which is static archives built without -fPIC. Build() refuses
all three now, each with a line that says why, before it makes a
directory or starts a thread.

The sysroot check sat in BuildStdPcm, which the linter calls as well.
There the error only dropped the compile command, so lint ran no AST
rule on a wasm configuration and reported it clean. It also refused
static libraries, which never link, fired after the shader and
external threads had started, and called a missing sysroot, or one
without the target, "no C++ exception support". It is part of the same
early check now, which first says when the sysroot or the target's
directory is missing, and asks for libunwind only where something
links.

C sources got only the signal.h define. setjmp.h refused to compile in
C, and a C object given -wasm-enable-sjlj through compileFlags used the
legacy exception encoding next to C++ objects in the standard one, a
mix engines refuse to load. They get WasmCompileFlags now, like the C++
units, a variant pass's flags included.

The baseline probes include simd: V8 and SpiderMonkey switch SIMD off
on x86 CPUs without SSE4.1, which virtual machines often present, and
such an engine passed the exception-handling probe only to fail
compiling the module; the manifest tests expect it on every entry. The
Flatpak pins WASI_SDK_EXCEPTIONS=DUAL, which
wasi-sdk 34 defaults to only with clang 23 or newer, and checks that
the eh/ libunwind got installed: the wasm tests skip without it, so CI
would otherwise pass a Flatpak whose every wasm build fails.

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

tests/WasmVariants/main.cpp

@@ -97,10 +97,11 @@ int main() {
             Check(j.find("\"label\":\"relaxed-simd\"") != std::string::npos, "manifest has relaxed-simd label");
             Check(j.find("\"wvhelper.relaxed-simd.wasm\"") != std::string::npos, "manifest has labelled url");
             Check(j.find("\"relaxed-simd\"") != std::string::npos, "manifest has relaxed-simd probe");
-            // Every file is built with exceptions, so every entry needs them
-            // besides its own features; the baseline needs nothing else.
-            Check(j.find("\"probes\":[\"relaxed-simd\",\"exception-handling\"]") != std::string::npos, "relaxed-simd entry also probes for exception handling");
-            Check(j.find("\"url\":\"wvhelper.wasm\",\"probes\":[\"exception-handling\"]") != std::string::npos, "baseline entry probes for exception handling only");
+            // Every file is built with SIMD128 and exceptions, so every entry
+            // needs them besides its own features; the baseline needs
+            // nothing else.
+            Check(j.find("\"probes\":[\"relaxed-simd\",\"simd\",\"exception-handling\"]") != std::string::npos, "relaxed-simd entry also probes for the baseline's features");
+            Check(j.find("\"url\":\"wvhelper.wasm\",\"probes\":[\"simd\",\"exception-handling\"]") != std::string::npos, "baseline entry probes for its own features only");
             // baseline fallback entry.
             Check(j.find("\"wvhelper.wasm\"") != std::string::npos, "manifest has baseline url");
             // The relaxed-simd variant must precede the baseline so the runtime