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Add support for generating and using gcc's native dependency files with
the GNU Assembler in addition to the existing C/C++ support.

When the gas and gccdeps tools are loaded, the configure step will test
whether gcc operating on an assembly file supports the -MMD argument.
If so, waf will pass the -MMD argument to .S files assembled with gcc
which will cause it to generate .d dependency files. Waf will then parse
those files for dependency information.

Note: This will only work for assembly files compiled through the gcc
frontend, not with GNU as directly. It also requires assembly files to
use the uppercase .S file extension.