gh-136976: Emscripten: Add _decimal and libmpdec

[hoodmane]

• Issue: Emscripten build doesn't include _decimal, _uuid, _ssl or _hashlib #136976

[skirpichev]

It still says on WASI build:

checking for --with-system-libmpdec... yes
checking for libmpdec >= 2.5.0... no
checking for --with-decimal-contextvar... yes

and then

The necessary bits to build these optional modules were not found:
_bz2                  _ctypes               _decimal           
_hashlib              _lzma                 _ssl               
_uuid                 _zstd                 readline           
zlib                                                           

[hoodmane]

Well yes, this shouldn't change the wasi build only the Emscripten build.

[hoodmane]

Closing in favor of #136997 because I accidentally opened this from my main branch. Sorry!