TESTSUITE: Fix Wconversion-null*.C tests for aarch64 -mabi=ilp32

TESTSUITE: Fix Wconversion-null*.C tests for aarch64 -mabi=ilp32

This fixes regressions for aarch64 with -mabi=ilp32 of the
Wconversion-null*.C tests, introduced by 92ea8e1bccc.

The "g (int)" declaration is required for that target where
sizeof(int) == sizeof(long) == sizeof(void *).

To handle the msp430/-mlarge case, an explicit declaration of
"g (__int20)" is required.

gcc/testsuite/ChangeLog:

	* g++.dg/warn/Wconversion-null-2.C: Add explicit declarations for l()
	and g() with int, long, long long and __int20 arguments.
	* g++.dg/warn/Wconversion-null.C: Likewise.