This patch fixes PR63596.

There is no need to push/pop all arguments registers. We only need to
push and pop those registers used. These use info is calculated by a
dedicated vaarg optimization tree pass "tree-stdarg", the backend should
honor it's analysis result.

For a simple testcase where vaarg declared but actually not used:

int
f (int a, ...)
{
   return a;
}

before this patch, we are generating:

f:
         sub     sp, sp, #192
         stp     x1, x2, [sp, 136]
         stp     x3, x4, [sp, 152]
	stp     x5, x6, [sp, 168]
         str     x7, [sp, 184]
         str     q0, [sp]
         str     q1, [sp, 16]
         str     q2, [sp, 32]
         str     q3, [sp, 48]
	str     q4, [sp, 64]
         str     q5, [sp, 80]
         str     q6, [sp, 96]
         str     q7, [sp, 112]
	add     sp, sp, 192
	ret

after this patch, it's optimized into:

f:
   	ret

OK for trunk?

2016-05-06  Jiong Wang  <jiong.wang@arm.com>
gcc/
   PR63596
   * config/aarch64/aarch64.c (aarch64_expand_builtin_va_start): Honor
   tree-stdarg analysis results.
   (aarch64_setup_incoming_varargs): Likewise.

gcc/testsuite/
   PR63596
   * gcc.target/aarch64/va_arg_1.c: New testcase.
   * gcc.target/aarch64/va_arg_2.c: Likewise.
   * gcc.target/aarch64/va_arg_3.c: Likewise.