This patch improves alloca alignment.  Currently alloca reserves
too much space as it aligns twice, and generates unnecessary stack
alignment code.

When the requested alignment is lower than the stack alignment, no
extra alignment is needed.  If the requested alignment is higher,
we need to increase the size by the difference of the requested 
alignment and the stack alignment.  As a result, the alloca alignment
is exactly as expected:

alloca (16):
	sub	sp, sp, #16
	mov	x1, sp

alloca (x):
	add	x0, x0, 15
	and	x0, x0, -16
	sub	sp, sp, x0
	mov	x0, sp

__builtin_alloca_with_align (x, 512):
	add	x0, x0, 63
	and	x0, x0, -16
	sub	sp, sp, x0
	add	x0, sp, 63
	and	x0, x0, -64

    gcc/
	* explow.c (get_dynamic_stack_size): Improve dynamic alignment.

From-SVN: r251713