Allow using MSVC intrinsics with older versions.

src/lj_def.h

@@ -242,6 +242,8 @@ static LJ_AINLINE uint32_t lj_getu32(const void *p)
 #define LJ_FASTCALL	__fastcall
 #endif
 
+#pragma intrinsic(_BitScanForward)
+#pragma intrinsic(_BitScanReverse)
 unsigned char _BitScanForward(uint32_t *, unsigned long);
 unsigned char _BitScanReverse(uint32_t *, unsigned long);
 unsigned long _byteswap_ulong(unsigned long);