Stack pointer alignment code incorrectly adds stack_size to sp instead
of subtracting it. It also does not take flat_argvp_envp_on_stack() into
account when calculating stack_size. This results in initial stack
pointer misalignment with certain set of command line arguments and
environment variables and correct alignment for the same binary with a
different set of arguments. This misalignment causes failures in the
following tests in the testsuite of gcc built for xtensa uclinux:

  gcc.dg/torture/vshuf-v64qi.c
  gcc.dg/torture/vshuf-v8sf.c
  gcc.dg/torture/vshuf-v8si.c

Reviewed-by: Laurent Vivier <lvivier@redhat.com>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>