Update default_estimated_poly_value prototype in targhooks.h

Update default_estimated_poly_value prototype in targhooks.h

commit 64432b680eab0bddbe9a4ad4798457cf6a14ad60
Author: Kyrylo Tkachov <kyrylo.tkachov@arm.com>
Date:   Thu Dec 17 18:02:37 2020 +0000

    vect, aarch64: Extend SVE vs Advanced SIMD costing decisions in vect_better_loop_vinfo_p

changed default_estimated_poly_value to

HOST_WIDE_INT
default_estimated_poly_value (poly_int64 x, poly_value_estimate_kind)
{
  return x.coeffs[0];
}

Update default_estimated_poly_value prototype in targhooks.h to match it.

	* targhooks.h (default_estimated_poly_value): Updated.