Allow to pass more sourcemap options to emcc

[lvlte]

It would be nice to be able to specify more sourcemap options to emcc, I'm thinking about -s options that maps to the following wasm-sourcemap.py options :

• --prefix
• --sources

emscripten/tools/wasm-sourcemap.py

Lines 32 to 45 in a9651ff

	def parse_args():
	parser = argparse.ArgumentParser(prog='wasm-sourcemap.py', description=__doc__)
	parser.add_argument('wasm', help='wasm file')
	parser.add_argument('-o', '--output', help='output source map')
	parser.add_argument('-p', '--prefix', nargs='*', help='replace source debug filename prefix for source map', default=[])
	parser.add_argument('-s', '--sources', action='store_true', help='read and embed source files from file system into source map')
	parser.add_argument('-l', '--load-prefix', nargs='*', help='replace source debug filename prefix for reading sources from file system (see also --sources)', default=[])
	parser.add_argument('-w', nargs='?', help='set output wasm file')
	parser.add_argument('-x', '--strip', action='store_true', help='removes debug and linking sections')
	parser.add_argument('-u', '--source-map-url', nargs='?', help='specifies sourceMappingURL section contest')
	parser.add_argument('--dwarfdump', help="path to llvm-dwarfdump executable")
	parser.add_argument('--dwarfdump-output', nargs='?', help=argparse.SUPPRESS)
	parser.add_argument('--basepath', help='base path for source files, which will be relative to this')
	return parser.parse_args()