message: Remove stack pointer from clobber list
  This fixes an issue when compiling with GCC 9.1:
  "In function 'void early_start_checks()': error: listing the stack pointer register 'sp' in a clobber list is deprecated [-Werror=deprecated]"
  Corresponds fix in upstream ARM CMSIS:
diff: @@ -161,7 +161,7 @@ __attribute__( ( always_inline ) ) __STATIC_INLINE uint32_t __get_PSP(void)
  */
  __attribute__( ( always_inline ) ) __STATIC_INLINE void __set_PSP(uint32_t topOfProcStack)
  {
  - __ASM volatile ("MSR psp, %0\n" : : "r" (topOfProcStack) : "sp");
  + __ASM volatile ("MSR psp, %0\n" : : "r" (topOfProcStack) : );
  }
  @@ -187,7 +187,7 @@ __attribute__( ( always_inline ) ) __STATIC_INLINE uint32_t __get_MSP(void)
  */
  __attribute__( ( always_inline ) ) __STATIC_INLINE void __set_MSP(uint32_t topOfMainStack)
  {
  - __ASM volatile ("MSR msp, %0\n" : : "r" (topOfMainStack) : "sp");
  + __ASM volatile ("MSR msp, %0\n" : : "r" (topOfMainStack) : );
  }
  