Use 16 byte stack alignment for NaCl on ARM

NaCl's ARM ABI uses 16 byte stack alignment, so set that in
ARMSubtarget.cpp.

Using 16 byte alignment exposes an issue in code generation in which a
varargs function leaves a 4 byte gap between the values of r1-r3 saved
to the stack and the following arguments that were passed on the
stack.  (Previously, this code only needed to support 4 byte and 8
byte alignment.)

With this issue, llc generated:

varargs_func:
        sub     sp, sp, #16
        push    {lr}
        sub     sp, sp, #12
        add     r0, sp, #16   // Should be 20
        stm     r0, {r1, r2, r3}
        ldr     r0, .LCPI0_0  // Address of va_list
        add     r1, sp, #16
        str     r1, [r0]
        bl      external_func

Fix the bug by checking for "Align > 4".  Also simplify the code by
using OffsetToAlignment(), and update comments.

Differential Revision: http://llvm-reviews.chandlerc.com/D2677