Fix "address will never be NULL" warning

Fix "address will never be NULL" warning

The mingw32 port is the only port to have TARGET_OVERRIDES_FORMAT_ATTRIBUTES
defined. When this macro is defined, it will never evaluate to NULL and produce
a warning: "error: the address of 'mingw_format_attribute_overrides' will never
be NULL [-Werror=address]"

  Also, when TARGET_OVERRIDES_FORMAT_ATTRIBUTES is defined,
TARGET_OVERRIDES_FORMAT_ATTRIBUTES_COUNT must be defined as well. Add that
requirement to the docs.

2022-09-07  Jan-Benedict Glaw  <jbglaw@lug-owl.de>

gcc/c-family/ChangeLog:
	* c-format.cc (convert_format_name_to_system_name): Fix warning.
gcc/ChangeLog:
	* doc/tm.texi.in (TARGET_OVERRIDES_FORMAT_ATTRIBUTES): Document requirement
	of TARGET_OVERRIDES_FORMAT_ATTRIBUTES_COUNT being defined as well.
	* doc/tm.texi: Regenerate.