riscv: Avoid clobbering register parameters in syscall

The riscv INTERNAL_SYSCALL macro might clobber the register
parameter if the argument itself might clobber any register (a function
call for instance).

This patch fixes it by using temporary variables for the expressions
between the register assignments (as indicated by GCC documentation,
6.47.5.2 Specifying Registers for Local Variables).

It is similar to the fix done for MIPS (bug 25523).

Checked with riscv64-linux-gnu-rv64imafdc-lp64d build.

(cherry picked from commit be74b42e)