loongarch: Add SLJIT_UPPER_BITS_SIGN_EXTENDED (#276)

Notice that in LoongArch V1.1 (LA664 and later), the
SLJIT_UPPER_BITS_IGNORED should also be true, however due to a
micro-architecture bug, 3A5000 (LA464) will needs the upper 32
bits of source register is sign extended while doing 32 bits
division (e.g. div.w), otherwise it will output garbage result.

See: https://loongson.github.io/LoongArch-Documentation/LoongArch-Vol1-EN.html
Chapter 1.5.1 Item 7

sljit_src/sljitConfigInternal.h

@@ -781,6 +781,7 @@ SLJIT_API_FUNC_ATTRIBUTE sljit_sw sljit_exec_offset(void *code);
 #define SLJIT_LOCALS_OFFSET_BASE 0
 #define SLJIT_MASKED_SHIFT 1
 #define SLJIT_MASKED_SHIFT32 1
+#define SLJIT_UPPER_BITS_SIGN_EXTENDED 1
 
 #elif (defined SLJIT_CONFIG_UNSUPPORTED && SLJIT_CONFIG_UNSUPPORTED)