Simplify flags in c compiling, fix NDK build warnings

Reduce the number of flags variables when compiling C by getting
rid of the separately-tracked IncludeDirs variables, and putting
them directly in GlobalFlags.  Also changes exportedIncludeDirs
to exportedFlags, which will allow exporting flags besides -I.

Also fixes a bug when building a module with the NDK and -Werror
by passing NDK exported include directories with -isystem instead
of -I, which will ignore warnings from those includes.

Also fixes a bug where the default includes were being inserted
into all cflags by the arm and arm64 toolchain modules.

Change-Id: Ice97f84e637b21d20c38c85b881afa315f9f92ae

cc/util.go

@@ -77,8 +77,6 @@ func ccFlagsToBuilderFlags(in CCFlags) builderFlags {
 		cppFlags:    strings.Join(in.CppFlags, " "),
 		yaccFlags:   strings.Join(in.YaccFlags, " "),
 		ldFlags:     strings.Join(in.LdFlags, " "),
-		ldLibs:      strings.Join(in.LdLibs, " "),
-		incFlags:    includeDirsToFlags(in.IncludeDirs),
 		nocrt:       in.Nocrt,
 		toolchain:   in.Toolchain,
 		clang:       in.Clang,