Fix test case failure on Linux s390x, which does not have the CoreGraphics module.

Author: bryanpkc

test/1_stdlib/tgmath.swift

@@ -7,7 +7,7 @@
   // where they doesn't have CoreGraphics module.
   #if arch(i386) || arch(arm)
     typealias CGFloat = Float
-  #elseif arch(x86_64) || arch(arm64) || arch(powerpc64) || arch(powerpc64le)
+  #elseif arch(x86_64) || arch(arm64) || arch(powerpc64) || arch(powerpc64le) || arch(s390x)
     typealias CGFloat = Double
   #endif
 #else