New CPU FE310 from SiFive based on RISC-V architecture

build: add makefile for RISC-V builds

Makefile for builds using RISC-V tools

Works around a regression in gcc 8.1.
See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85606.

CFLAGS_CPU is used in ASFLAGS, but assembler doesn't know that
parameter. Moving to CFLAGS uses it only for the C compiler.

This allows providing 'vectors.o' by another module than 'cpu' by overwriting
VECTORS_O variable.

Causes compilation error on Clang

Some headers in the tool chain cause compilation errors with Clang in
assembly mode.

 - Copied the relevant parts of mipshal.mk from the toolchain
 - Cleaned up CFLAGS and more in Makefile.include
 - Added empty syscalls implementation which can be used to interface
   with the UHI system
 - Added a note on why -std=gnu99 is necessary for this CPU presently