gdb/gdbserver: change shared set_tdesc_osabi to take gdb_osabi

gdb/gdbserver: change shared set_tdesc_osabi to take gdb_osabi

There is a single declaration of set_tdesc_osabi that is shared
between gdbserver/ and gdb/, this declaration takes a 'const char *'
argument which is the string representing an osabi.

Then in gdb/ we have an overload of set_tdesc_osabi which takes an
'enum gdb_osabi'.

In this commit I change the shared set_tdesc_osabi to be the version
which takes an 'enum gdb_osabi', and I remove the version which takes
a 'const char *'.  All users of set_tdesc_osabi are updated to pass an
'enum gdb_osabi'.

The features/ code, which is generated from the xml files, requires a
new function to be added to osabi.{c,h} which can return a string
representation of an 'enum gdb_osabi'.  With that new function in
place the features/ code is regenerated.

This change is being made to support the next commit.  In the next
commit gdbserver will be updated to call set_tdesc_osabi in more
cases.  The problem is that gdbserver stores the osabi as a string.
The issue here is that a typo in the gdbserver/ code might go
unnoticed and result in gdbserver sending back an invalid osabi
string.

To fix this we want gdbserver to pass an 'enum gdb_osabi' to the
set_tdesc_osabi function.  With that requirement in place it seems to
make sense if all calls to set_tdesc_osabi pass an 'enum gdb_osabi'.

There should be no user visible changes after this commit.

Approved-By: Luis Machado <luis.machado@arm.com>
Approved-By: Simon Marchi <simon.marchi@efficios.com>