AP_GPS: pragma pack is needed on ARM

we rely on GPS data structures having byte alignment. Luckily GCC can
cope with this.

libraries/AP_GPS/AP_GPS_MTK.h

@@ -25,7 +25,7 @@ public:
     static bool _detect(uint8_t );
 
 private:
-// XXX this is being ignored by the compiler #pragma pack(1)
+    #pragma pack(push,1)
     struct diyd_mtk_msg {
         int32_t latitude;
         int32_t longitude;
@@ -36,7 +36,7 @@ private:
         uint8_t fix_type;
         uint32_t utc_time;
     };
-// #pragma pack(pop)
+    #pragma pack(pop)
     enum diyd_mtk_fix_type {
         FIX_NONE = 1,
         FIX_2D = 2,

libraries/AP_GPS/AP_GPS_MTK16.h

@@ -24,7 +24,7 @@ public:
     static bool _detect(uint8_t );
 
 private:
-// XXX this is being ignored by the compiler #pragma pack(1)
+    #pragma pack(push,1)
     struct diyd_mtk_msg {
         int32_t latitude;
         int32_t longitude;
@@ -37,7 +37,7 @@ private:
         uint32_t utc_time;
         uint16_t hdop;
     };
-// #pragma pack(pop)
+    #pragma pack(pop)
     enum diyd_mtk_fix_type {
         FIX_NONE = 1,
         FIX_2D = 2,

libraries/AP_GPS/AP_GPS_MTK19.h

@@ -28,7 +28,7 @@ public:
     static bool 		_detect(uint8_t );
 
 private:
-// XXX this is being ignored by the compiler #pragma pack(1)
+	#pragma pack(push,1)
     struct diyd_mtk_msg {
         int32_t latitude;
         int32_t longitude;
@@ -41,7 +41,7 @@ private:
         uint32_t utc_time;
         uint16_t hdop;
     };
-// #pragma pack(pop)
+	#pragma pack(pop)
     enum diyd_mtk_fix_type {
         FIX_NONE = 1,
         FIX_2D = 2,

libraries/AP_GPS/AP_GPS_SIRF.h

@@ -23,7 +23,7 @@ public:
 	static bool         _detect(uint8_t data);
 
 private:
-// XXX this is being ignored by the compiler #pragma pack(1)
+    #pragma pack(push,1)
     struct sirf_geonav {
         uint16_t fix_invalid;
         uint16_t fix_type;
@@ -61,7 +61,7 @@ private:
         uint8_t hdop;
         uint8_t mode_info;
     };
-// #pragma pack(pop)
+    #pragma pack(pop)
     enum sirf_protocol_bytes {
         PREAMBLE1 = 0xa0,
         PREAMBLE2 = 0xa2,

libraries/AP_GPS/AP_GPS_UBLOX.h

@@ -39,7 +39,7 @@ public:
 
 private:
     // u-blox UBX protocol essentials
-// XXX this is being ignored by the compiler #pragma pack(1)
+	#pragma pack(push,1)
     struct ubx_header {
         uint8_t preamble1;
         uint8_t preamble2;
@@ -124,7 +124,8 @@ private:
         uint32_t speed_accuracy;
         uint32_t heading_accuracy;
     };
-// // #pragma pack(pop)
+	#pragma pack(pop)
+
     enum ubs_protocol_bytes {
         PREAMBLE1 = 0xb5,
         PREAMBLE2 = 0x62,