bfd: support for NT_386_TLS notes

bfd: support for NT_386_TLS notes

In a later commit I'd like to add support to GDB for including the
NT_386_TLS note in the core files that GDB creates (using 'gcore'
command).

To achieve this we need some standard boilerplate code added to bfd.

The only part of this patch which I think needs consideration is the
name I selected for the pseudo section to hold the note contents when
a core file is loaded.  I chose '.reg-i386-tls'.  The '.reg' prefix is
the standard used by most other pseudo sections, and the '-i386-tls'
suffix seemed to match the note name, though I added the 'i' to
'i386', instead of just using '.reg-386-tls'.  I thought 'i386' seemed
clearer.

There's no test included here, but when I merge the NT_386_TLS
creation to GDB it will depend on this and act as a test.  I plan to
post that work to the GDB list once this patch is merged.