For x86_64, gcc 7.2 under Amazon Linux AMI sets its path to x86_64-amazon-linux.

gcc 7.2 under Amazon Linux AMI sets its paths to x86_64-amazon-linux. Adding 
this triple to the list of search, plus a test case to cover this.

The patch fixes the following bug reported in bugzilla:

https://bugs.llvm.org/show_bug.cgi?id=35992

Reviewers: echristo

Differential Revision: https://reviews.llvm.org/D46230

llvm-svn: 337811
diff --git a/clang/lib/Driver/ToolChains/Gnu.cpp b/clang/lib/Driver/ToolChains/Gnu.cpp
index 1ecf127..2c83598 100644
--- a/clang/lib/Driver/ToolChains/Gnu.cpp
+++ b/clang/lib/Driver/ToolChains/Gnu.cpp
@@ -1834,7 +1834,8 @@
       "x86_64-pc-linux-gnu",    "x86_64-redhat-linux6E",
       "x86_64-redhat-linux",    "x86_64-suse-linux",
       "x86_64-manbo-linux-gnu", "x86_64-linux-gnu",
-      "x86_64-slackware-linux", "x86_64-unknown-linux"};
+      "x86_64-slackware-linux", "x86_64-unknown-linux",
+      "x86_64-amazon-linux"};
   static const char *const X32LibDirs[] = {"/libx32"};
   static const char *const X86LibDirs[] = {"/lib32", "/lib"};
   static const char *const X86Triples[] = {