[ARM GlobalISel] Support G_FMUL for s32 and s64

[ARM GlobalISel] Support G_FMUL for s32 and s64

TableGen already generates code for selecting a G_FMUL, so we only need
to add a test for that part.

For the legalizer and reg bank select, we do the same thing as the other
floating point binary operators: either mark as legal if we have a FP
unit or lower to a libcall, and map to the floating point registers.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@318910 91177308-0d34-0410-b5e6-96231b3b80d8