Build: Add back TFM_CODE_COVERAGE setting in GNUARM toolchain

TFM_CODE_COVERAGE was removed by accident from toolchain_GNUARM.cmake.
Add it back.

Change-Id: I2cdd5f57963d36b094c05ffcc930de371e725600
Signed-off-by: David Hu <david.hu@arm.com>
diff --git a/toolchain_GNUARM.cmake b/toolchain_GNUARM.cmake
index 2ed640b..9cb741c 100644
--- a/toolchain_GNUARM.cmake
+++ b/toolchain_GNUARM.cmake
@@ -48,7 +48,7 @@
         -mthumb
         -nostdlib
         -std=c99
-        $<$<BOOL:${TFM_DEBUG_SYMBOLS}>:-g>
+        $<$<OR:$<BOOL:${TFM_DEBUG_SYMBOLS}>,$<BOOL:${TFM_CODE_COVERAGE}>>:-g>
     )
 endmacro()