cmake: gnuarm: Use .ld as suffix for preprocessed file instead of .o

Use .ld instead of .o as the suffix of the preprocessed linker script.

Using the incorrect extension .o will confuse both readers and tooling
like editors and diff viewers.

Change-Id: I0a412426c40717e086c9bc4330cbb27a4cc810bd
Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>
diff --git a/toolchain_GNUARM.cmake b/toolchain_GNUARM.cmake
index e100ab6..067101d 100644
--- a/toolchain_GNUARM.cmake
+++ b/toolchain_GNUARM.cmake
@@ -124,6 +124,7 @@
         set_source_files_properties(${SCATTER_FILE_PATH}
             PROPERTIES
             LANGUAGE C
+            KEEP_EXTENSION True # Don't use .o extension for the preprocessed file
         )
     endforeach()
 
@@ -273,4 +274,4 @@
                             ${ORIG_TARGET}
                             "${LIB_LIST}"
     )
-endmacro()
\ No newline at end of file
+endmacro()