rs6000: Remove old psabi warnings

rs6000: Remove old psabi warnings

Long ago we were forced to make some small ABI breaks to correct errors
in the implementation, and we added warning messages for the changes
from GCC 4.9 to GCC 5.  Enough time has passed that these are now just
irritants, so let's remove them.  Also clean up associated macros using
rs6000_special_adjust_field_align_p, which has been always returning
false for a long time.

2021-05-18  Bill Schmidt  <wschmidt@linux.ibm.com>

gcc/
	* config/rs6000/freebsd64.h (ADJUST_FIELD_ALIGN): Remove call to
	rs6000_special_adjust_field_align_p.
	* config/rs6000/linux64.h (ADJUST_FIELD_ALIGN): Likewise.
	* config/rs6000/rs6000-call.c (rs6000_function_arg_boundary):
	Remove ABI warning.
	(rs6000_function_arg): Likewise.
	* config/rs6000/rs6000-protos.h
	(rs6000_special_adjust_field_align_p): Remove prototype.
	* config/rs6000/rs6000.c (rs6000_special_adjust_field_align_p):
	Remove.
	* config/rs6000/sysv4.h (ADJUST_FIELD_ALIGN): Remove call to
	rs6000_special_adjust_field_align_p.

gcc/testsuite/
	* gcc.target/powerpc/ppc64-abi-warn-1.c: Remove.
	* gcc.target/powerpc/ppc64-abi-warn-2.c: Remove.
	* gcc.target/powerpc/ppc64-abi-warn-3.c: Remove.