[Driver] Disable -faddrsig on Gentoo by default

Gentoo supports combining clang toolchain with GNU binutils, and many
users actually do that.  As -faddrsig is not supported by GNU strip,
this results in a lot of warnings.  Disable it by default and let users
enable it explicitly if they want it; with the intent of reevaluating
when the underlying feature becomes standarized.

See also: https://bugs.gentoo.org/667854

Differential Revision: https://reviews.llvm.org/D56047

llvm-svn: 350028
diff --git a/clang/test/lit.cfg.py b/clang/test/lit.cfg.py
index d105f8d..e61b4bb 100644
--- a/clang/test/lit.cfg.py
+++ b/clang/test/lit.cfg.py
@@ -190,3 +190,6 @@
 macOSSDKVersion = lit.util.findPlatformSdkVersionOnMacOS(config, lit_config)
 if macOSSDKVersion is not None:
     config.available_features.add('macos-sdk-' + macOSSDKVersion)
+
+if os.path.exists('/etc/gentoo-release'):
+    config.available_features.add('gentoo')