Rewrote riscv-canonicalize-gen.py using xml file

Rewrote riscv-canonicalize-gen.py using xml file

Since catching-syscalls was added, there had been added files containing
syscalls in xml format. As for now riscv-canonicalize-syscall-gen.py uses
glibc for generating, it may be not so comfortable. I changed this
script for reusing newly generated riscv-linux.xml file. Also, I renamed
riscv64_canonicalize_syscall to riscv_linux_canonicalize_syscall as only 64
system is supported in linux. This will simplify the possible further
generalization of this script to other architectures.

Approved-By: Simon Marchi <simon.marchi@efficios.com>