[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/ToolChains/Gnu.cpp b/clang/lib/Driver/ToolChains/Gnu.cpp
index ad702b6..3755673 100644
--- a/clang/lib/Driver/ToolChains/Gnu.cpp
+++ b/clang/lib/Driver/ToolChains/Gnu.cpp
@@ -1877,7 +1877,8 @@
 
   static const char *const RISCV32LibDirs[] = {"/lib", "/lib32"};
   static const char *const RISCVTriples[] = {"riscv32-unknown-linux-gnu",
-                                             "riscv64-unknown-linux-gnu"};
+                                             "riscv64-unknown-linux-gnu",
+                                             "riscv32-unknown-elf"};
 
   static const char *const SPARCv8LibDirs[] = {"/lib32", "/lib"};
   static const char *const SPARCv8Triples[] = {"sparc-linux-gnu",