Apply WERROR only to stage2 HC options

Otherwise validate unnecessarily may fail during the stage1 build when
booting with an older compiler.

Test Plan: Try it

Reviewers: thomie, austin

Reviewed By: thomie, austin

Differential Revision: https://phabricator.haskell.org/D1390

ghc.mk

@@ -143,7 +143,8 @@ include mk/warnings.mk
 # (Optional) build-specific configuration
 include mk/custom-settings.mk
 SRC_CC_OPTS     += $(WERROR)
-SRC_HC_OPTS     += $(WERROR)
+GhcStage2HcOpts += $(WERROR)
+GhcLibHcOpts    += $(WERROR)
 
 # -----------------------------------------------------------------------------
 # Check for inconsistent settings, after reading mk/build.mk.