[3.14] gh-148557: Use em-config to locate trampoline clang (GH-148556) (#150482)

When CC is wrapped by ccache, the Emscripten trampoline rule cannot derive the
matching clang path by treating CC as a single executable path. Query the active
LLVM toolchain path with em-config instead.
(cherry picked from commit 1310d2c)

Co-authored-by: Clément Péron <peron.clem@gmail.com>

Author: miss-islington

Makefile.pre.in

@@ -3112,7 +3112,7 @@ Python/asm_trampoline.o: $(srcdir)/Python/asm_trampoline.S
 
 Python/emscripten_trampoline_inner.wasm: $(srcdir)/Python/emscripten_trampoline_inner.c
 	# emcc has a path that ends with emsdk/upstream/emscripten/emcc, we're looking for emsdk/upstream/bin/clang.
-	$$(dirname $$(dirname $(CC)))/bin/clang -o $@ $< -mgc -O2 -Wl,--no-entry -Wl,--import-table -Wl,--import-memory -target wasm32-unknown-unknown -nostdlib
+	$$(em-config LLVM_ROOT)/clang -o $@ $< -mgc -O2 -Wl,--no-entry -Wl,--import-table -Wl,--import-memory -target wasm32-unknown-unknown -nostdlib
 
 Python/emscripten_trampoline_wasm.c: Python/emscripten_trampoline_inner.wasm
 	$(PYTHON_FOR_REGEN) $(srcdir)/Platforms/emscripten/prepare_external_wasm.py $< $@ getWasmTrampolineModule