Handle fixed-point types in amd64_classify

gdb.ada/fixed_points_function.exp was failing when compiled with
gnat-llvm.  Debugging showed that this was a gdb bug that was hidden
by differences in the DWARF generated by gcc and gnat-llvm.

In particular, gcc emitted a DW_TAG_subrange_type (a subrange of the
fixed-point type) for the function's parameter type, whereas gnat-llvm
used a fixed-point type directly.

Then, the test failed because amd64_classify recognizes subrange types
but not fixed-point types.  Under the hood, fixed-point types are
really just integers, so the fix is to handle these directly in
amd64_classify.

Approved-By: Christina Schimpe <christina.schimpe@intel.com>