riscv64: implement signum{float|double} and copySign{float|double} intrinsics

Performance improvement:
copySign(double)   -29%
copySign(float)    -33%
signum(double)     -20%
signum(float)      -22%

Test: testrunner.py --target --64 --ndebug --optimizing
Change-Id: I3b69a4d35a0b37e5debed9f16c3a757a4204387e