Fix and simplify build of static libraries.

gprconfig was generating no Archive_Builder attribute for
mips-linux-gnu, powerpc64le-linux-gnu and many other architectures,
because the regular expressions defining specific architectures and
the fallback were not covering all cases.  Ensure that all cases are
covered by the fallback stanza.

Replace many handwritten copies with variable substitutions.

Merge i686-pc-mingw32 and x86_64-pc-mingw32 as already done for other
architectures with aliases.

Merge native x86 into the default stanza.

This merge request once was
AdaCore/gprbuild#67.

Author: asarhaddon