util/cacheflush.c: Update cache flushing mechanism for Emscripten

util/cacheflush.c: Update cache flushing mechanism for Emscripten

Although __builtin___clear_cache is used to flush the instruction cache for
a specified memory region, this operation doesn't apply to wasm, as its
memory isn't executable. Moreover, Emscripten does not support this builtin
and fails to compile it with the following error.

> fatal error: error in backend: llvm.clear_cache is not supported on wasm

To resolve this, this commit removes the call to __builtin___clear_cache for
Emscripten build.

Signed-off-by: Kohei Tokunaga <ktokunaga.mail@gmail.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Link: https://lore.kernel.org/r/2926a798fa52a3a5b11c3df4edd1643d2b7cdcb9.1745820062.git.ktokunaga.mail@gmail.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>