[RISCV] Add driver for riscv32-unknown-elf baremetal target

Summary:
This patch adds a driver for the baremetal RISC-V target (i.e. riscv32-unknown-elf). For reference, D39963 added basic target info and added support for riscv32-linux-unknown-elf.

Patch by: asb (Alex Bradbury)

Reviewers: efriedma, phosek, apazos, espindola, mgrang

Reviewed By: mgrang

Subscribers: jrtc27, rogfer01, MartinMosbeck, brucehoult, the_o, rkruppe, emaste, mgorny, arichardson, rbar, johnrusso, simoncook, jordy.potman.lists, sabuasal, niosHD, kito-cheng, shiva0217, zzheng, edward-jones, mgrang, cfe-commits

Differential Revision: https://reviews.llvm.org/D46822

llvm-svn: 338385
diff --git a/clang/lib/Driver/Driver.cpp b/clang/lib/Driver/Driver.cpp
index 1dfcacc..952a716 100644
--- a/clang/lib/Driver/Driver.cpp
+++ b/clang/lib/Driver/Driver.cpp
@@ -37,6 +37,7 @@
 #include "ToolChains/NetBSD.h"
 #include "ToolChains/OpenBSD.h"
 #include "ToolChains/PS4CPU.h"
+#include "ToolChains/RISCV.h"
 #include "ToolChains/Solaris.h"
 #include "ToolChains/TCE.h"
 #include "ToolChains/WebAssembly.h"
@@ -4399,6 +4400,10 @@
       case llvm::Triple::avr:
         TC = llvm::make_unique<toolchains::AVRToolChain>(*this, Target, Args);
         break;
+      case llvm::Triple::riscv32:
+      case llvm::Triple::riscv64:
+        TC = llvm::make_unique<toolchains::RISCVToolChain>(*this, Target, Args);
+        break;
       default:
         if (Target.getVendor() == llvm::Triple::Myriad)
           TC = llvm::make_unique<toolchains::MyriadToolChain>(*this, Target,