Fix broken attr-alloc_size-10.c for avr

Fix broken attr-alloc_size-10.c for avr

Integer promotion combined with equal sizes for short and int (16 bits)
causes overflow warnings when expanding the TEST macro for USHRT_MAX.

Fix by explicitly disabling overflow warnings for targets with !int32plus.

gcc/testsuite/

2017-04-25  Senthil Kumar Selvaraj  <senthil_kumar.selvaraj@atmel.com>

	* gcc.dg/attr-alloc_size-10.c: Ignore overflow warnings
	for targets with int size less than 32 bits.

From-SVN: r247203