[Triple] Add isRISCV function

This matches isARM, isThumb, isAArch64 and similar helpers. Future commits
which clean-up code that currently checks for Triple::riscv32 ||
Triple::riscv64.

Differential Revision: https://reviews.llvm.org/D54215
Patch by Simon Cook.
Test case added by Alex Bradbury.

llvm-svn: 365327
diff --git a/llvm/unittests/ADT/TripleTest.cpp b/llvm/unittests/ADT/TripleTest.cpp
index b0f089a..556d9b9 100644
--- a/llvm/unittests/ADT/TripleTest.cpp
+++ b/llvm/unittests/ADT/TripleTest.cpp
@@ -876,11 +876,13 @@
   EXPECT_FALSE(T.isArch16Bit());
   EXPECT_TRUE(T.isArch32Bit());
   EXPECT_FALSE(T.isArch64Bit());
+  EXPECT_TRUE(T.isRISCV());
 
   T.setArch(Triple::riscv64);
   EXPECT_FALSE(T.isArch16Bit());
   EXPECT_FALSE(T.isArch32Bit());
   EXPECT_TRUE(T.isArch64Bit());
+  EXPECT_TRUE(T.isRISCV());
 }
 
 TEST(TripleTest, BitWidthArchVariants) {