AVR: Use INT_N to built-in define __int24.

AVR: Use INT_N to built-in define __int24.

This patch uses the INT_N interface to define __int24 in avr-modes.def.

Since the testsuite uses -Wpedantic and __int24 is a C/C++ extension,
uses of __int24 and __uint24 is now marked as __extension__.

	PR target/118329
gcc/
	* config/avr/avr-modes.def: Add INT_N (PSI, 24).
	* config/avr/avr.cc (avr_init_builtin_int24)
	<__int24>: Remove definition.
	<__uint24>: Adjust definition to INT_N interface.
gcc/testsuite/
	* gcc.target/avr/pr115830-add.c (__int24, __uint24): Add __extension__
	to respective typedefs.
	* gcc.target/avr/pr115830-sub-ext.c: Same.
	* gcc.target/avr/pr115830-sub.c: Same.
	* gcc.target/avr/torture/get-mem.c: Same.
	* gcc.target/avr/torture/set-mem.c: Same.
	* gcc.target/avr/torture/ifelse-c.h: Same.
	* gcc.target/avr/torture/ifelse-d.h: Same.
	* gcc.target/avr/torture/ifelse-q.h: Same.
	* gcc.target/avr/torture/ifelse-r.h: Same.
	* gcc.target/avr/torture/int24-mul.c: Same.
	* gcc.target/avr/torture/pr109907-2.c: Same.
	* gcc.target/avr/torture/pr61443.c: Same.
	* gcc.target/avr/torture/pr63633-ice-mult.c: Same.
	* gcc.target/avr/torture/shift-l-u24.c: Same.
	* gcc.target/avr/torture/shift-r-i24.c: Same.
	* gcc.target/avr/torture/shift-r-u24.c: Same.
	* gcc.target/avr/torture/add-extend.c: Same.
	* gcc.target/avr/torture/sub-extend.c: Same.
	* gcc.target/avr/torture/sub-zerox.c: Same.
	* gcc.target/avr/torture/test-gprs.h: Same.