Makefile: Check -Wno-stringop-truncation is supported

The cross compiler on my system throws a fit as it does not understand
this option:

 cc1: error: unrecognized command line option '-Wno-stringop-truncation' [-Werror]

Fixes: 918b723 ("Add -Wno-stringop-truncation for GCC8")
Signed-off-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Stewart Smith <stewart@linux.ibm.com>

Makefile.main

@@ -31,7 +31,7 @@ CWARNS := -Wall -Wundef -Wstrict-prototypes -Wno-trigraphs \
 	  -Wwrite-strings -Wcast-align \
 	  -Winit-self \
 	  -Wframe-larger-than=1024 \
-	  -Wno-stringop-truncation \
+	  $(call try-cflag,$(CC),--Wno-stringop-truncation) \
 	  -Werror
 
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