arch-riscv: Add missing system calls

arch-riscv: Add missing system calls

This patch adds all system calls present in riscv-gnu-toolchain at
commit hash 65cb174. Many of them do not have implementations in gem5, so
they are just placeholders.

[Remove the variadic macro and replace it with a function that has
default arguments because variadic macros may not be portable across
compilers]
[Remove spaces around default arguments of createSyscall for better
style]
[Switch from using a loose function to using SyscallDesc's new
constructor]

Change-Id: Iff97c689109121b39df423c72d0e79c6366e31b9
Reviewed-on: https://gem5-review.googlesource.com/5322
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Alec Roelke <ar4jc@virginia.edu>