add EM_OPENRISC again to elf.h

it was changed to EM_OR1K in 200d15479c
as that was meant to be the official name, but glibc and the latest
gabi spec still uses the EM_OPENRISC name:
http://www.sco.com/developers/gabi/latest/ch4.eheader.html
binutils defines both macros so we should do the same for backward
compatibility.

include/elf.h

@@ -210,6 +210,7 @@ typedef struct {
 #define EM_MN10200	90
 #define EM_PJ		91
 #define EM_OR1K		92
+#define EM_OPENRISC	92
 #define EM_ARC_A5	93
 #define EM_XTENSA	94
 #define EM_AARCH64	183