Add additional CI via GitHub Actions. #79

Job	Run time
build_emsdk (debug)	43s
build_emsdk (debug, -msimd128)	43s
build_emsdk (debug, -msse -msse2 -msimd128)	46s
build_emsdk (debug, -msse -msse2 -msse3 -msse4 -msimd128)	45s
build_emsdk (debugoptimized)	44s
build_emsdk (debugoptimized, -msimd128)	43s
build_emsdk (debugoptimized, -msse -msse2 -msimd128)	47s
build_emsdk (debugoptimized, -msse -msse2 -msse3 -msse4 -msimd128)	42s
build_emsdk (release)	42s
build_emsdk (release, -msimd128)	38s
build_emsdk (release, -msse -msse2 -msimd128)	40s
build_emsdk (release, -msse -msse2 -msse3 -msse4 -msimd128)	43s
build_emsdk (minsize)	42s
build_emsdk (minsize, -msimd128)	42s
build_emsdk (minsize, -msse -msse2 -msimd128)	45s
build_emsdk (minsize, -msse -msse2 -msse3 -msse4 -msimd128)	45s
	11m 30s

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