[Driver][RISCV] Add RedHat Linux RISC-V triple

[luismarques]

Summary

Adds the RedHat Linux triple to the list of 64-bit RISC-V triples. Without this the gcc libraries wouldn't be found by clang on a redhat/fedora system, as the search list included /usr/lib/gcc/riscv64-redhat-linux-gnu but the correct path didn't include the -gnu suffix.

[lenary]

LGTM!

[lebedev.ri]

passing-by remark: tests missing

[luismarques]

Adds a test (using the RISC-V Fedora 31 paths).

[luismarques]

Correct paths.

clang/lib/Driver/ToolChains/Gnu.cpp

/*static*/ void Generic_GCC::GCCInstallationDetector::CollectLibDirsAndTriples(
   static const char *const RISCV32LibDirs[] = {"/lib32", "/lib"};
   static const char *const RISCV32Triples[] = {"riscv32-unknown-linux-gnu",
                                                "riscv32-linux-gnu",
                                                "riscv32-unknown-elf"};
   static const char *const RISCV64LibDirs[] = {"/lib64", "/lib"};
   static const char *const RISCV64Triples[] = {"riscv64-unknown-linux-gnu",
                                                "riscv64-linux-gnu",
                                                "riscv64-unknown-elf",
+                                               "riscv64-redhat-linux",
                                                "riscv64-suse-linux"};
 
   static const char *const SPARCv8LibDirs[] = {"/lib32", "/lib"};
   static const char *const SPARCv8Triples[] = {"sparc-linux-gnu",
                                                "sparcv8-linux-gnu"};
   static const char *const SPARCv9LibDirs[] = {"/lib64", "/lib"};
   static const char *const SPARCv9Triples[] = {"sparc64-linux-gnu",
                                                "sparcv9-linux-gnu"};