Abort build for stm32f4xx platform if cross-compiler wasn't set

[forGGe]

It is possible to trigger TheCore build using host compiler and get some strange errors:

[ 31%] Building C object core/platform/stm32f4xx/SPL/CMakeFiles/spl.dir/src/stm32f4xx_cryp_tdes.c.o
/home/executor/projects/thecore/demo-example/core/platform/stm32f4xx/SPL/src/stm32f4xx_cryp_tdes.c: In function ‘CRYP_TDES_ECB’:
/home/executor/projects/thecore/demo-example/core/platform/stm32f4xx/SPL/src/stm32f4xx_cryp_tdes.c:108:25: error: cast from pointer to integer of different size [-Werror=pointer-to-int-cast]
   uint32_t keyaddr    = (uint32_t)Key;
                         ^
/home/executor/projects/thecore/demo-example/core/platform/stm32f4xx/SPL/src/stm32f4xx_cryp_tdes.c:109:25: error: cast from pointer to integer of different size [-Werror=pointer-to-int-cast]
   uint32_t inputaddr  = (uint32_t)Input;
                         ^
/home/executor/projects/thecore/demo-example/core/platform/stm32f4xx/SPL/src/stm32f4xx_cryp_tdes.c:110:25: error: cast from pointer to integer of different size [-Werror=pointer-to-int-cast]
   uint32_t outputaddr = (uint32_t)Output;
                         ^
/home/executor/projects/thecore/demo-example/core/platform/stm32f4xx/SPL/src/stm32f4xx_cryp_tdes.c:131:53: error: cast to pointer from integer of different size [-Werror=int-to-pointer-cast]
   TDES_CRYP_KeyInitStructure.CRYP_Key1Left = __REV(*(uint32_t*)(keyaddr));
                                                     ^
/home/executor/projects/thecore/demo-example/core/platform/stm32f4xx/SPL/src/stm32f4xx_cryp_tdes.c:133:53: error: cast to pointer from integer of different size [-Werror=int-to-pointer-cast]
   TDES_CRYP_KeyInitStructure.CRYP_Key1Right= __REV(*(uint32_t*)(keyaddr));
                                                     ^
/home/executor/projects/thecore/demo-example/core/platform/stm32f4xx/SPL/src/stm32f4xx_cryp_tdes.c:135:53: error: cast to pointer from integer of different size [-Werror=int-to-pointer-cast]
   TDES_CRYP_KeyInitStructure.CRYP_Key2Left = __REV(*(uint32_t*)(keyaddr));
                                                     ^
/home/executor/projects/thecore/demo-example/core/platform/stm32f4xx/SPL/src/stm32f4xx_cryp_tdes.c:137:53: error: cast to pointer from integer of different size [-Werror=int-to-pointer-cast]
   TDES_CRYP_KeyInitStructure.CRYP_Key2Right= __REV(*(uint32_t*)(keyaddr));
                                                     ^
/home/executor/projects/thecore/demo-example/core/platform/stm32f4xx/SPL/src/stm32f4xx_cryp_tdes.c:139:53: error: cast to pointer from integer of different size [-Werror=int-to-pointer-cast]
   TDES_CRYP_KeyInitStructure.CRYP_Key3Left = __REV(*(uint32_t*)(keyaddr));
                                                     ^
/home/executor/projects/thecore/demo-example/core/platform/stm32f4xx/SPL/src/stm32f4xx_cryp_tdes.c:141:53: error: cast to pointer from integer of different size [-Werror=int-to-pointer-cast]
   TDES_CRYP_KeyInitStructure.CRYP_Key3Right= __REV(*(uint32_t*)(keyaddr));
                                                     ^
/home/executor/projects/thecore/demo-example/core/platform/stm32f4xx/SPL/src/stm32f4xx_cryp_tdes.c:159:18: error: cast to pointer from integer of different size [-Werror=int-to-pointer-cast]
     CRYP_DataIn(*(uint32_t*)(inputaddr));
                  ^
/home/executor/projects/thecore/demo-example/core/platform/stm32f4xx/SPL/src/stm32f4xx_cryp_tdes.c:161:18: error: cast to pointer from integer of different size [-Werror=int-to-pointer-cast]
     CRYP_DataIn(*(uint32_t*)(inputaddr));
                  ^

Reason is that cross-compiler wasn't set. It is hard to notice though.

[forGGe]

Implemented in #171