gdbsupport: move gdb_file_up to its own file

gdbsupport: move gdb_file_up to its own file

The following patches wants to change gdb_fopen_cloexec and
gdb_mkostemp_cloexec to return a scoped_fd.  Doing this causes a cyclic
include between scoped_fd.h and filestuff.h, that both want to include
each other.  scoped_fd.h includes filestuff.h because of the
scoped_fd::to_file method's return value.  filestuff.h would then
include scoped_fd.h for gdb_fopen_cloexec's and gdb_mkostemp_cloexec's
return values.

To fix that, move gdb_file_up to its own file, gdb_file.h.

Change-Id: Ic82a48914b2aacee8f14af535b7469245f88b93d