[gdb/build] Disable attribute nonnull

With trunk gcc (12.0) we're running into a -Werror=nonnull-compare build
breaker in gdb, which caused a broader review of the usage of the nonnull
attribute.

The current conclusion is that it's best to disable this.  This is explained
at length in the gdbsupport/common-defs.h comment.

Tested by building with trunk gcc.

gdb/ChangeLog:

2021-07-29  Tom de Vries  <tdevries@suse.de>

	* gdbsupport/common-defs.h (ATTRIBUTE_NONNULL): Disable.