Consider allowing multiple cores to be used in make to speed up compilation from scratch

[coatless]

I think only the tools/ llvm part of the compilation is allowing jobs/commands to run simultaneously. As a result, the build times from scratch are a bit time-intensive. Would it be possible to add -j$(NPROC) after $(MAKE) to allow using as many cores as possible?

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Note, that there are some parallelized portions already in use. Specifically, in the llvm part due to the use of ninja.

cmake -G Ninja -S /Users/ronin/Documents/GitHub/oss/webr/tools/flang/llvm-project/llvm -B /Users/ronin/Documents/GitHub/oss/webr/tools/flang/build \
	  -DCMAKE_INSTALL_PREFIX=/Users/ronin/Documents/GitHub/oss/webr/host \
	  -DCMAKE_BUILD_TYPE=MinSizeRel \
	  -DLLVM_DEFAULT_TARGET_TRIPLE="wasm32-unknown-emscripten" \
	  -DLLVM_TARGETS_TO_BUILD="WebAssembly" \
	  -DLLVM_ENABLE_PROJECTS="clang;flang;mlir" \

Also, in a separate file related to dragonegg that doesn't seem to be called, we have the desired $(MAKE) command using -j$(NPROC):

webr/tools/dragonegg/Makefile

Line 40 in 2f3b467

$(MAKE) -j$(NPROC)