[ARM] GlobalISel: Lower double precision FP args

For the hard float calling convention, we just use the D registers.

For the soft-fp calling convention, we use the R registers and move values
to/from the D registers by means of G_SEQUENCE/G_EXTRACT. While doing so, we
make sure to honor the endianness of the target, since the CCAssignFn doesn't do
that for us.

For pure soft float targets, we still bail out because we don't support the
libcalls yet.

llvm-svn: 295295