riscv: fix up architecture-dependent klibc files.
Also, get rid of nonsense builds for mcmodel=small and such. That was
never needed.
To test for riscv you can do things like this:
ARCH=riscv \
CC=/home/rminnich/projects/riscv/riscv-tools/bin/riscv64-unknown-elf-gcc \
LD=/home/rminnich/projects/riscv/riscv-tools/bin/riscv64-unknown-elf-ld build
to build just cmds, for testing, add, e.g.;
sys/src/cmd/cmds.json
to the command
Signed-off-by: Ronald G. Minnich <rminnich@gmail.com>