Use more generic and more accurate tests for LP64 and endianness.

Index: src/include/janet.h
--- src/include/janet.h.orig
+++ src/include/janet.h
@@ -103,44 +103,18 @@ extern "C" {
 #endif
 
 /* Check 64-bit vs 32-bit */
-#if ((defined(__x86_64__) || defined(_M_X64)) \
-     && (defined(JANET_POSIX) || defined(JANET_WINDOWS))) \
-    || (defined(_WIN64)) /* Windows 64 bit */ \
-    || (defined(__ia64__) && defined(__LP64__)) /* Itanium in LP64 mode */ \
-    || defined(__alpha__) /* DEC Alpha */ \
-    || (defined(__sparc__) && defined(__arch64__) || defined (__sparcv9)) /* BE */ \
-    || defined(__s390x__) /* S390 64-bit (BE) */ \
-    || (defined(__ppc64__) || defined(__PPC64__)) \
-    || defined(__aarch64__) /* ARM 64-bit */ \
-    || (defined(__riscv) && (__riscv_xlen == 64)) /* RISC-V 64-bit */
+#if defined(__LP64__) || defined(_LP64)
 #define JANET_64 1
 #else
 #define JANET_32 1
 #endif
 
-/* Check big endian */
-#if defined(__LITTLE_ENDIAN__) || \
-    (defined(__BYTE_ORDER__) && (__BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__))
-/* If we know the target is LE, always use that - e.g. ppc64 little endian
- * defines the __LITTLE_ENDIAN__ macro in the ABI spec, so we can rely
- * on that and if that's not defined, fall back to big endian assumption
- */
+/* Check endianness */
+#include <endian.h>
+#if BYTE_ORDER == LITTLE_ENDIAN
 #define JANET_LITTLE_ENDIAN 1
-#elif defined(__MIPSEB__) /* MIPS 32-bit */ \
-    || defined(__ppc__) || defined(__PPC__) /* CPU(PPC) - PowerPC 32-bit */ \
-    || defined(__powerpc__) || defined(__powerpc) || defined(__POWERPC__) \
-    || defined(_M_PPC) || defined(__PPC) \
-    || defined(__ppc64__) || defined(__PPC64__) /* PowerPC 64-bit */ \
-    || defined(__sparc)   /* Sparc 32bit */  \
-    || defined(__sparc__) /* Sparc 64-bit */ \
-    || defined(__s390x__) /* S390 64-bit */ \
-    || defined(__s390__)  /* S390 32-bit */ \
-    || defined(__ARMEB__) /* ARM big endian */ \
-    || ((defined(__CC_ARM) || defined(__ARMCC__)) /* ARM RealView compiler */ \
-        && defined(__BIG_ENDIAN))
-#define JANET_BIG_ENDIAN 1
 #else
-#define JANET_LITTLE_ENDIAN 1
+#define JANET_BIG_ENDIAN 1
 #endif
 
 /* Limits for converting doubles to 64 bit integers */