[efi] Allow for building with older versions of elf.h system header

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/util/elf2efi.c

 #ifndef R_ARM_THM_JUMP24
 #define R_ARM_THM_JUMP24 30
 #endif
+#ifndef R_ARM_V4BX
+#define R_ARM_V4BX 40
+#endif
 
 /* Seems to be missing from elf.h */
 #ifndef R_AARCH64_NULL