Make devel/riscv64-binutils a slave of devel/binutils.

Since the riscv support was merged into the upstream binutils repository
prior to the 2.30 release, switch the riscv64-binutils port over to the
standard binutils port rather than building from an alternate branch of
binutils 2.29.

Reviewed by: lwhsu
Sponsored by: DARPA / AFRL
Differential Revision: https://reviews.freebsd.org/D16053