[binutils-gdb] Silence gcc-8 warnings

[sergiodj+buildbot@sergiodj.net]

*** TEST RESULTS FOR COMMIT d99b4b92c8ed0f7ef98f370bbf65a360ed66ad7b ***

Author: Alan Modra <amodra@gmail.com>
Branch: master
Commit: d99b4b92c8ed0f7ef98f370bbf65a360ed66ad7b

Silence gcc-8 warnings

This seems to work with gcc-8 and a bunch of prior gcc versions I tested.

	* elf-linux-core.h: Disable gcc-8 string truncation warning.
	* elf.c (elfcore_write_prpsinfo): Likewise.