limit RISC-V FindMatchLength optimizations to 64-bit

Add `__riscv_xlen == 64` check to ensure the optimized FindMatchLength
routine is only enabled on 64-bit RISC-V platforms.

Signed-off-by: Zhou Yunfei <xdzyf2012@gmail.com>

snappy-internal.h

diff --git a/snappy-internal.h b/snappy-internal.h
index 00b2db5..9be0542 100644
--- a/snappy-internal.h
+++ b/snappy-internal.h
@@ -194,7 +194,7 @@
 // riscv and little-endian cpu choose this routinue can be done faster too.
 #if !SNAPPY_IS_BIG_ENDIAN && \
     (defined(__x86_64__) || defined(_M_X64) || defined(ARCH_PPC) || \
-     defined(ARCH_ARM) || defined(__riscv))
+     defined(ARCH_ARM) || (defined(__riscv) && (__riscv_xlen == 64)))
 static inline std::pair<size_t, bool> FindMatchLength(const char* s1,
                                                       const char* s2,
                                                       const char* s2_limit,