testsuite: generalized field-merge tests for <32-bit int [PR118025]

Explicitly convert constants to the desired types, so as to not elicit
warnings about implicit truncations, nor execution errors, on targets
whose ints are narrower than 32 bits.


for  gcc/testsuite/ChangeLog

	PR testsuite/118025
	* gcc.dg/field-merge-1.c: Convert constants to desired types.
	* gcc.dg/field-merge-3.c: Likewise.
	* gcc.dg/field-merge-4.c: Likewise.
	* gcc.dg/field-merge-5.c: Likewise.
	* gcc.dg/field-merge-11.c: Likewise.
	* gcc.dg/field-merge-17.c: Don't mess with padding bits.