[feature request] Option for a custom compiler wrapper path for emmake / emconfigure / emcmake (EM_COMPILER_WRAPPER?)

**Original title**: [feature request] emcc option for copy-pasting a third-party binary instead of compiling #12256

While porting a recent texlive to emscripten (https://github.com/vadimkantorov/busytex), I found useful the following pattern: overriding [`CC="python3 ccskip.py /some/native/binary1 /some/native/binary2 -- emcc"`](https://github.com/vadimkantorov/busytex/blob/master/ccskip.py) when calling `make`. This allows skipping compiling some intermediate build-time tools and using copy-pasting their native versions instead.

Is there another recommended way for achieveing this? If not, I propose adding some option to emcc that would achieve the same. and would be more discoverable.

Some things that I had to copy-over:
```
CCSKIP_wasm_icu = python3 $(ROOT)/ccskip.py "$(ROOT)/build/native/texlive/libs/icu/icu-build/bin/icupkg" "$(ROOT)/build/native/texlive/libs/icu/icu-build/bin/pkgdata" --
CCSKIP_wasm_freetype2 = python3 $(ROOT)/ccskip.py $(ROOT)/build/native/texlive/libs/freetype2/ft-build/apinames --
CCSKIP_wasm_xetex = python3 $(ROOT)/ccskip.py $(addprefix $(ROOT)/build/native/texlive/texk/web2c, ctangle otangle tangle tangleboot ctangleboot tieweb2c) $(addprefix $(ROOT)/build/native/texlive/texk/web2c/web2c, fixwrites makecpool splitup web2c) --
```