checkasm/arm: preserve the stack alignment checkasm_checked_call

The stack used by checkasm_checked_call_vfp was a multiple of 4 when the
checked function is called. AAPCS requires a double word (8 byte)
aligned stack public interfaces. Since both calls are public interfaces
the stack is misaligned when the checked is called.

Might fix the SIGBUS error in the armv7-linux-clang-3.7 fate config.
