Fix sloppy PR126536 test

This test failed with

FAIL: gcc.dg/pr126536-1.c (test for excess errors)
Excess errors:
gcc.dg/pr126536-1.c:37:5: warning: overflow in conversion from 'long int' to 'int' changes value from '100000' to '-31072' [-Woverflow]
gcc.dg/pr126536-1.c:37:5: warning: empty range specified
gcc.dg/pr126536-1.c:39:5: warning: overflow in conversion from 'long int' to 'int' changes value from '200000' to '3392' [-Woverflow]
FAIL: gcc.dg/pr126536-1.c execution test

As it's obviously requiring int32plus in the argument of f.
Hence use __INT32_TYPE__ instead of int.

gcc/testsuite/
	PR testsuite/126536
	* gcc.dg/pr126536-1.c (f): Pass a as __INT32_TYPE__.