arch: arm: Remove redundant check and fix rounding

This patch removes the redundant stack alignment check being done.  The
stack definition macros enforce the alignment requirements via the
__align() directives.

In addition, fix the rounding down of the psp to be correct.  The
actual initial stack pointer is the end of the stack minus the size of
the __esf structure.  Rounding down after the subtraction will get us
to the correct offset.

Signed-off-by: Andy Gross <andy.gross@linaro.org>