Add support for Hexagon architecture #8

Job	Run time
build_iwasm	1m 16s
build_llvm_libraries / build_llvm_libraries	13s
build_wamrc	41s
spec_test (-s spec -b -P, fast-interp)	7m 4s
spec_test (-s spec -b -S -P, fast-interp)	11m 31s
spec_test (-s spec -b -P, classic-interp)	7m 15s
spec_test (-s spec -b -P, aot)	50s
spec_test (-s spec -b -S -P, classic-interp)	20s
	29m 10s

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