bfd: add elfcore_write_file_note

Adds a trivial wrapper over elfcore_write_note, primarily to be more
consistent with other ELF note helper functions and highlight NT_FILE as
one of notes handled by gdb.

bfd/ChangeLog:
2020-12-17  Mihails Strasuns  <mihails.strasuns@intel.com>

	* bfd-elf.h (elfcore_write_file_note): New function.
	* elf.c (elfcore_write_file_note): New function.

gdb/ChangeLog:
2020-12-17  Mihails Strasuns  <mihails.strasuns@intel.com>

	* linux-tdep.c (linux_make_mappings_corefile_notes): Start using
	elfcore_write_file_note.