fix riscv64 a_cas inline asm operand sign extension

fix riscv64 a_cas inline asm operand sign extension

This patch adds an explicit cast to the int arguments passed to the
inline asm used in the RISC-V's implementation of `a_cas`, to ensure
that they are properly sign extended to 64 bits. They aren't
automatically sign extended by Clang, and GCC technically also doesn't
guarantee that they will be sign extended.