Add -mcpu=mvp flag to Wasm builds for compatibility

Starting from clang 16, the default Wasm features used silently include
sign-extension which is not part of MVP and is not supported by some old
browsers/devices like iPhone 6. This currently only affects
meshopt_encoder module which is typically not used on device, but a
future C++ or a compiler code change could silently introduce these
instructions in a way that is difficult to test.

For now just disable this since it's not helpful for performance or code
size. We could revisit this in a few years, but clang's policy of
updating features seems irresponsible as it is not actually favoring
compatibility by default.

This change doesn't rebuild js/ binaries, that can happen separately.

Makefile

@@ -40,6 +40,7 @@ WASIROOT?=$(WASI_SDK)/share/wasi-sysroot
 
 WASM_FLAGS=--target=wasm32-wasi --sysroot=$(WASIROOT)
 WASM_FLAGS+=-O3 -DNDEBUG -nostartfiles -nostdlib -Wl,--no-entry -Wl,-s
+WASM_FLAGS+=-mcpu=mvp # make sure clang doesn't use post-MVP features like sign extension
 WASM_FLAGS+=-fno-slp-vectorize -fno-vectorize -fno-unroll-loops
 WASM_FLAGS+=-Wl,-z -Wl,stack-size=24576 -Wl,--initial-memory=65536
 WASM_EXPORT_PREFIX=-Wl,--export