Move EM_RISCV into elf-em.h

This should never have been inside our arch port to begin with, it's
just a relic from when we were maintaining out of tree patches.

Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Paul Walmsley <paul.walmsley@sifive.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Tested-by: David Abdurachmanov <david.abdurachmanov@gmail.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>

arch/riscv/include/asm/elf.h

@@ -16,9 +16,6 @@
 #include <asm/auxvec.h>
 #include <asm/byteorder.h>
 
-/* TODO: Move definition into include/uapi/linux/elf-em.h */
-#define EM_RISCV	0xF3
-
 /*
  * These are used to set parameters in the core dumps.
  */