Bug 1483778 - Skip LTO during the profile-generate phase of PGO. r=froydnj

When both LTO and PGO are enabled, there is no point LTO'ing during the
first phase of PGO.

config/config.mk

@@ -199,6 +199,11 @@ INCLUDES = \
 
 include $(MOZILLA_DIR)/config/static-checking-config.mk
 
+ifdef MOZ_PROFILE_GENERATE
+MOZ_LTO_CFLAGS :=
+MOZ_LTO_LDFLAGS :=
+endif
+
 LDFLAGS		= $(MOZ_LTO_LDFLAGS) $(COMPUTED_LDFLAGS) $(PGO_LDFLAGS) $(MK_LDFLAGS)
 
 COMPILE_CFLAGS	= $(MOZ_LTO_CFLAGS) $(COMPUTED_CFLAGS) $(PGO_CFLAGS) $(_DEPEND_CFLAGS) $(MK_COMPILE_DEFINES)