xcompile: detect and use RISCV binaries

RISCV is a new architecture. This change simply setups up xcompile
to detect and use RISCV compilers if they are found.

Change-Id: Iad1a88ef2e3c8dd1e601549aeca26fb29b2bc7ae
Signed-off-by: Ronald G. Minnich <rminnich@gmail.com>
Reviewed-on: http://review.coreboot.org/7023
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <patrick@georgi-clan.de>