COFF: avoid modifications over C_FILE filename aux entries.

Commit e86fc4a5 ("PR 28447: implement multiple parameters for .file
on XCOFF") introduces C_FILE entries which can store additional
information.
However, some modifications are needed by them but not by the original
C_FILE entries, usually representing the filename.
This patch ensures that filename entries are kept as is, in order to
protect targets not supporting the additional entries.

	* coffgen.c (coff_write_symbol): Protect filename entries
	(coff_write_symbols): Likewise.
	(coff_print_symbol): Likewise.