AP_GPS: replace utility functions with compiler builtins

[peterbarker]

Tested with this patch:

--- a/libraries/AP_GPS/GPS_Backend.cpp
+++ b/libraries/AP_GPS/GPS_Backend.cpp
@@ -157,6 +157,21 @@ AP_GPS_Backend::inject_data(const uint8_t *data, uint16_t len)
 
 void AP_GPS_Backend::_detection_message(char *buffer, const uint8_t buflen) const
 {
+    const int32_t values[] { 0, 1, INT32_MAX, INT32_MIN, 17, 123456789};
+    for (auto value : values) {
+        if (swap_int32(value) != (int32_t)htobe32(value)) {
+            abort();
+        }
+    }
+    gcs().send_text(MAV_SEVERITY_INFO, "swap32 OK");
+    const int16_t values16[] { 0, 1, INT16_MAX, INT16_MIN, 17, -17};
+    for (auto value : values16) {
+        if (swap_int16(value) != (int16_t) htobe16(value)) {
+            abort();
+        }
+    }
+    gcs().send_text(MAV_SEVERITY_INFO, "swap16 OK");
+
     const uint8_t instance = state.instance;
     const struct AP_GPS::detect_state dstate = gps.detect_state[instance];
 

[peterbarker]

Board               AP_Periph  blimp  bootloader  copter  heli  iofirmware  plane  rover  sub
Durandal                       8      *           8       8                 8      8      8
HerePro             *                                                                     
Hitec-Airspeed      *                                                                     
KakuteH7-bdshot                0      *           -16     -24               0      -24    0
MatekF405                      *      *           *       *                 *      *      *
Pixhawk1-1M-bdshot             *                  *       *                 *      *      *
f103-QiotekPeriph   *                                                                     
f303-Universal      *                                                                     
iomcu                                                           *                         
revo-mini                      0      *           -16     -24               0      -24    0
skyviper-v2450                                    *                                       

[amilcarlucas]

Good catch