riscv --enable-targets=all on 32-bit host

Attempting to build --enable-targets=all on a 32-bit host results in a
number of errors like the following.

eelf32lriscv.o: in function `gldelf32lriscv_after_allocation':
eelf32lriscv.c:98: undefined reference to `bfd_elf32_riscv_restart_relax_sections'

That's due to needing --enable-64-bit-bfd to get the riscv BFD support
built.

	* Makefile.am (ALL_EMULATION_SOURCES): Move riscv files to..
	(ALL_64_EMULATION_SOURCES): ..here.
	* Makefile.in: Regenerate.