Update Makefile.{singlesrc,multisrc} to use X_TARGET_FLAGS

The LNT test harness handles TARGET_FLAGS specially, and prevents other
users of the variable from functioning as intended, i.e.   "TARGET_FLAGS
+= -maltivec" does not work as intended.
X_TARGET_FLAGS exists to work around this, but was not utilized in
Makefile.{singlesrc,multisrc}.   Update those makefiles to use
X_TARGET_FLAGS, and update Altivec/Makefile appropriately.




git-svn-id: https://llvm.org/svn/llvm-project/test-suite/trunk@174712 91177308-0d34-0410-b5e6-96231b3b80d8