Support grouping static libraries

LLVM has complicated static library layering that sometimes changes.
The make solution was to list all the static libraries twice, but
Soong dedups the list.  Add a group_static_libs flag to allow
surrounding the static libs with -Wl,--start-group and
-Wl,--end-group.

Test: mmma -j external/llvm
Change-Id: Ic08a183d7def9c9249d4a3014760759f16b68d04

cc/util.go

@@ -100,6 +100,8 @@ func flagsToBuilderFlags(in Flags) builderFlags {
 		toolchain:   in.Toolchain,
 		clang:       in.Clang,
 		tidy:        in.Tidy,
+
+		groupStaticLibs: in.GroupStaticLibs,
 	}
 }