Fix clash of gcc toolchains in hexagon driver regression tests.

If clang was configured with a custom gcc toolchain (either by using GCC_INSTALL_PREFIX in cmake or the equivalent configure command), the path to the custom gcc toolchain path takes precedence to the one specified by -ccc-install-dir. This causes several regression tests to fail as they will be using an unexpected path. Adding the switch --gcc-toolchain="" in each test command is not enough as the hexagon toolchain implementation in the driver is not evaluating this argument. This commit modifies the hexagon toolchain to take the --gcc-toolchain="" argument into account when deciding the toolchain path, similarly to what is already done for other targets toolchains. Additionally, the faulty regression tests are modified in order to --gcc-toolchain="" be passed to the commands.

llvm-svn: 221535
diff --git a/clang/lib/Driver/Tools.cpp b/clang/lib/Driver/Tools.cpp
index 24d602f..ca705c1 100644
--- a/clang/lib/Driver/Tools.cpp
+++ b/clang/lib/Driver/Tools.cpp
@@ -5249,8 +5249,8 @@
   const std::string MarchSuffix = "/" + MarchString;
   const std::string G0Suffix = "/G0";
   const std::string MarchG0Suffix = MarchSuffix + G0Suffix;
-  const std::string RootDir = toolchains::Hexagon_TC::GetGnuDir(D.InstalledDir)
-                              + "/";
+  const std::string RootDir =
+      toolchains::Hexagon_TC::GetGnuDir(D.InstalledDir, Args) + "/";
   const std::string StartFilesDir = RootDir
                                     + "hexagon/lib"
                                     + (buildingLib