Fix static array size in gcc.dg/vect/vect-simd-20.c

Fix static array size in gcc.dg/vect/vect-simd-20.c

10000 / 78 is strictly greater than 128 so we will
actually do 128+1 strides in foo() for s == 78 and p[]
needs to be dimensioned accordingly.

2021-12-20  Olivier Hainque  <hainque@adacore.com>

gcc/testsuite/
	* gcc.dg/vect/vect-simd-20.c: Fix size of p[]
	to accommodate the number of strides performed
	by foo() for s == 78.