Define SUFFIX_DATA

This allows RISC OS abstract data files with filetype ffd to be referenced
identically whether native or cross-compiling (it expands to an empty string
when used natively).

GNUmakefiles/StdTools

@@ -8,6 +8,7 @@ INCLUDED_STDTOOLS = YES
 
 ifneq (Host,${APCS})
 GNUTOOLPREFIX = arm-unknown-riscos-
+SUFFIX_DATA     = ,ffd
 SUFFIX_MODULE   = ,ffa
 SUFFIX_SPRITE   = ,ff9
 SUFFIX_ABSOLUTE = ,ff8