manywarnings: accommodate GCC 9.0-pre: remove -Wchkp and -Wabi
* build-aux/gcc-warning.spec: Add them here, each with an explanation.
* m4/manywarnings.m4: Remove them.
Otherwise, building coreutils, I would see this:
cc1: error: deprecated command line option '-Wchkp' [-Werror]
cc1: error: -Wabi won't warn about anything [-Werror=abi]
cc1: note: -Wabi warns about differences from the most up-to-date ABI,\
which is also used by default
cc1: note: use e.g. -Wabi=11 to warn about changes from GCC 7