gdb: Improve cache matching criteria for the bfd cache.

gdb: Improve cache matching criteria for the bfd cache.

Within gdb open bfd objects are reused where possible if an attempt is
made to reopen a file that is already being debugged.  To spot if the on
disc file has changed gdb currently examines the mtime of the file and
compares it to the mtime of the open bfd in the cache.

A problem exists when the on disc file is being rapidly regenerated, as
happens, for example, with automated testing.  In some cases the file is
generated so quickly that the mtime appears not to change, while the on
disc file has changed.

This patch extends the bfd cache to also hold the file size of the file,
the inode of the file, and the device id of the file; gdb can then
compare filename, file size, mtime, inode, and device id to determine if
an existing bfd object can be reused.

gdb/ChangeLog:

	* gdb_bfd.c (struct gdb_bfd_data): Add size, inode, and device id
	field.
	(struct gdb_bfd_cache_search): Likewise.
	(eq_bfd): Compare the size, inode, and device id fields.
	(gdb_bfd_open): Initialise the size, inode, and device id fields.
	(gdb_bfd_ref): Likewise.
	(gdb_bfd_unref): Likewise.