Add FreeBSD/riscv architecture.

Add FreeBSD/riscv architecture.

Support for collecting and supplying general purpose and floating
point register sets is provided along with signal frame unwinding.

FreeBSD only supports RV64 currently, so while some provision is made
for RV32 in the general-purpose register set, the changes have only
been tested on RV64.

gdb/ChangeLog:

	* Makefile.in (ALL_TARGET_OBS): Add riscv-fbsd-tdep.o.
	(HFILES_NO_SRCDIR): Add riscv-fbsd-tdep.h.
	(ALLDEPFILES): Add riscv-fbsd-tdep.c.
	* NEWS: Mention new FreeBSD/riscv target.
	* configure.tgt: Add riscv*-*-freebsd*.
	* riscv-fbsd-tdep.c: New file.
	* riscv-fbsd-tdep.h: New file.