common/swap: Simplify swap function ifdefs

Including every OS' own built-in byte swapping functions is kind of
undesirable, since it adds yet another build path to ensure compilation
succeeds on.

Given we only support clang, GCC, and MSVC for the time being, we can
utilize their built-in functions directly instead of going through the
OS's API functions.

This shrinks the overall code down to just

if (msvc)
  use msvc's functions
else if (clang or gcc)
  use clang/gcc's builtins
else
  use the slow path

src/common/swap.h

@@ -21,11 +21,6 @@
 
 #if defined(_MSC_VER)
 #include <cstdlib>
-#elif defined(__linux__)
-#include <byteswap.h>
-#elif defined(__Bitrig__) || defined(__DragonFly__) || defined(__FreeBSD__) ||                     \
-    defined(__NetBSD__) || defined(__OpenBSD__)
-#include <sys/endian.h>
 #endif
 #include <cstring>
 #include "common/common_types.h"
@@ -62,58 +57,30 @@
 namespace Common {
 
 #ifdef _MSC_VER
-inline u16 swap16(u16 _data) {
-    return _byteswap_ushort(_data);
-}
-inline u32 swap32(u32 _data) {
-    return _byteswap_ulong(_data);
-}
-inline u64 swap64(u64 _data) {
-    return _byteswap_uint64(_data);
-}
-#elif __linux__
-inline u16 swap16(u16 _data) {
-    return bswap_16(_data);
-}
-inline u32 swap32(u32 _data) {
-    return bswap_32(_data);
-}
-inline u64 swap64(u64 _data) {
-    return bswap_64(_data);
-}
-#elif __APPLE__
-inline __attribute__((always_inline)) u16 swap16(u16 _data) {
-    return (_data >> 8) | (_data << 8);
+inline u16 swap16(u16 data) {
+    return _byteswap_ushort(data);
 }
-inline __attribute__((always_inline)) u32 swap32(u32 _data) {
-    return __builtin_bswap32(_data);
+inline u32 swap32(u32 data) {
+    return _byteswap_ulong(data);
 }
-inline __attribute__((always_inline)) u64 swap64(u64 _data) {
-    return __builtin_bswap64(_data);
+inline u64 swap64(u64 data) {
+    return _byteswap_uint64(data);
 }
-#elif defined(__Bitrig__) || defined(__OpenBSD__)
+#elif defined(__clang__) || defined(__GNUC__)
+#if defined(__Bitrig__) || defined(__OpenBSD__)
 // redefine swap16, swap32, swap64 as inline functions
 #undef swap16
 #undef swap32
 #undef swap64
-inline u16 swap16(u16 _data) {
-    return __swap16(_data);
-}
-inline u32 swap32(u32 _data) {
-    return __swap32(_data);
-}
-inline u64 swap64(u64 _data) {
-    return __swap64(_data);
-}
-#elif defined(__DragonFly__) || defined(__FreeBSD__) || defined(__NetBSD__)
-inline u16 swap16(u16 _data) {
-    return bswap16(_data);
+#endif
+inline u16 swap16(u16 data) {
+    return __builtin_bswap16(data);
 }
-inline u32 swap32(u32 _data) {
-    return bswap32(_data);
+inline u32 swap32(u32 data) {
+    return __builtin_bswap32(data);
 }
-inline u64 swap64(u64 _data) {
-    return bswap64(_data);
+inline u64 swap64(u64 data) {
+    return __builtin_bswap64(data);
 }
 #else
 // Slow generic implementation.