initial RISC-V support (#2614)

Unlike related PR #2344 that simply warns about unsupported FTZ, this PR
attempts to correctly handle FTZ on RISC-V.
RISC-V 'f' extension does not support any way to enable/disable flushing
subnormals to zero, implementations are required to always support
subnormals. Therefore this PR re-uses FTZ handling code from PPC, where
flushing also has to be explicitly performed.

test_common/harness/testHarness.cpp

@@ -1409,6 +1409,8 @@ void PrintArch(void)
     vlog("ARCH:\tWindows\n");
 #elif defined(__mips__)
     vlog("ARCH:\tmips\n");
+#elif defined(__riscv)
+    vlog("ARCH:\tRISC-V\n");
 #else
 #error unknown arch
 #endif