Reapply r322168 (partially, by br):

o Replace riscv with riscv
o Replace riscv64 with (riscv && riscv_xlen == 64)

This is required to support new GCC 7.1 compiler.
This is compatible with current GCC 6.1 compiler.

RISC-V is extensible ISA and the idea here is to have built-in define
per each extension, so together with __riscv we will have some subset
of these as well (depending on -march string passed to compiler):

riscv_compressed
riscv_atomic
riscv_mul
riscv_div
riscv_muldiv
riscv_fdiv
riscv_fsqrt
riscv_float_abi_soft
riscv_float_abi_single
riscv_float_abi_double
riscv_cmodel_medlow
riscv_cmodel_medany
riscv_cmodel_pic
riscv_xlen

Reviewed by: ngie
Sponsored by: DARPA, AFRL

Differential Revision: https://reviews.freebsd.org/D11901