[Driver][Gnu] Support -nolibc flag

This can be used to disable libc linking. This flag is supported by
GCC since version 9 as well as some Clang target toolchains.

Differential Revision: https://reviews.llvm.org/D58326

llvm-svn: 354210
diff --git a/clang/lib/Driver/ToolChains/Gnu.cpp b/clang/lib/Driver/ToolChains/Gnu.cpp
index e94a895..6eb8fe2 100644
--- a/clang/lib/Driver/ToolChains/Gnu.cpp
+++ b/clang/lib/Driver/ToolChains/Gnu.cpp
@@ -518,7 +518,8 @@
       if (Args.hasArg(options::OPT_fsplit_stack))
         CmdArgs.push_back("--wrap=pthread_create");
 
-      CmdArgs.push_back("-lc");
+      if (!Args.hasArg(options::OPT_nolibc))
+        CmdArgs.push_back("-lc");
 
       // Add IAMCU specific libs, if needed.
       if (IsIAMCU)