Support 32 bit and 64 bit decoding with one binary

It is possible to configure the build process such that decoding of 32
bit and 64 bit instructions can be chosen at runtime using an additional
parameter of the decode function. The header file is now entirely
architecture-independent and no longer required any previous defines.

Decoding x86-64 still requires a 64-bit pointer size.

decode.h

@@ -5,10 +5,6 @@
 #include <stddef.h>
 #include <stdint.h>
 
-#if defined(ARCH_X86_64) && __SIZEOF_POINTER__ < 8
-#error "Decoding x86-64 requires a 64-bit architecture"
-#endif
-
 #ifndef ssize_t
 #define ssize_t intptr_t
 #endif
@@ -22,6 +18,13 @@ enum
 #undef DECODE_TABLE_MNEMONICS
 #undef MNEMONIC
 
+enum DecodeMode {
+    DECODE_64 = 0,
+    DECODE_32 = 1,
+};
+
+typedef enum DecodeMode DecodeMode;
+
 enum RegIndex {
     RI_AL = 0,
     RI_CL,
@@ -40,7 +43,6 @@ enum RegIndex {
     RI_BP,
     RI_SI,
     RI_DI,
-#if defined(ARCH_X86_64)
     RI_R8,
     RI_R9,
     RI_R10,
@@ -49,12 +51,9 @@ enum RegIndex {
     RI_R13,
     RI_R14,
     RI_R15,
-#endif
 
     // EIP cannot be encoded in Protected/Compatibility Mode
-#if defined(ARCH_X86_64)
     RI_IP = 0x10,
-#endif
 
     RI_ES = 0,
     RI_CS,
@@ -82,13 +81,11 @@ enum PrefixSet
     PREFIX_LOCK = 1 << 4,
     PREFIX_REPNZ = 1 << 5,
     PREFIX_REP = 1 << 6,
-#if defined(ARCH_X86_64)
     PREFIX_REX = 1 << 7,
     PREFIX_REXB = 1 << 8,
     PREFIX_REXX = 1 << 9,
     PREFIX_REXR = 1 << 10,
     PREFIX_REXW = 1 << 11,
-#endif
     PREFIX_ESC_NONE = 0 << 13,
     PREFIX_ESC_0F = 1 << 13,
     PREFIX_ESC_0F38 = 2 << 13,
@@ -144,12 +141,9 @@ typedef struct Instr Instr;
 #define INSTR_HAS_REPNZ(instr) ((instr)->prefixes & PREFIX_REPNZ)
 #define INSTR_HAS_LOCK(instr) ((instr)->prefixes & PREFIX_LOCK)
 #define INSTR_HAS_ADDRSZ(instr) ((instr)->prefixes & PREFIX_ADDRSZ)
-
-#if defined(ARCH_X86_64)
 #define INSTR_HAS_REX(instr) ((instr)->prefixes & PREFIX_REX)
-#endif
 
-int decode(const uint8_t* buffer, int len, Instr* out_instr);
+int decode(const uint8_t* buffer, int len, DecodeMode mode, Instr* out_instr);
 void instr_format(const Instr* instr, char buffer[128]);
 void instr_print(const Instr* instr) __attribute__((deprecated));