riscv: Update the gcc built-in macro __riscv__

The architecture is determined by __riscv__ in the LZ4_copy8 function
(located in src / commonlib / lz4_wrapper.c). __riscv exists in gcc7.1.1.
But __riscv__ does not exist.

Change-Id: I38fd41da9afd76c254f0c3d6984579c3790e5792
Signed-off-by: XiangWang <wxjstz@126.com>
Reviewed-on: https://review.coreboot.org/20125
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Andrew Waterman <aswaterman@gmail.com>
Reviewed-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>

src/commonlib/lz4_wrapper.c

@@ -67,7 +67,7 @@ static void LZ4_copy8(void *dst, const void *src)
 			: "=m"(*(uint32_t *)(dst + 4))
 			: [x1]"r"(x1), [dst]"r"(dst));
 	#endif
-#elif defined(__riscv__)
+#elif defined(__riscv)
 	/* RISC-V implementations may trap on any unaligned access. */
 	int i;
 	for (i = 0; i < 8; i++)