Fix define MBED_EXCLUSIVE_ACCESS for Apple M1 (ARM64) computers

The new Apple M1 Mac computers are ARM based. When compiling and running
unit tests on an M1 Mac, the architecture is defined as __arm__

An extra check for __aarch64__ is needed to set MBED_EXCLUSIVE_ACCESS to 0U for the M1
Mac

If not, compilation fails with "Unknown ARM architecture for exclusive access" error

Author: ladislas

platform/include/platform/mbed_atomic.h

@@ -88,6 +88,8 @@ typedef enum mbed_memory_order {
 #endif
 #elif (__ARM_ARCH_6M__ == 1U)
 #define MBED_EXCLUSIVE_ACCESS      0U
+#elif defined __aarch64__ // Apple M1 Mac
+#define MBED_EXCLUSIVE_ACCESS      0U
 #else
 #error "Unknown ARM architecture for exclusive access"
 #endif // __ARM_ARCH_xxx