gdbserver: make arch and osabi names gdb::unique_xmalloc_ptr<char>

gdbserver: make arch and osabi names gdb::unique_xmalloc_ptr<char>

Convert target_desc::arch and target_desc::osabi from 'const char*' to
gdb::unique_xmalloc_ptr<char>.  This also allows us to remove the user
defined ~target_desc destructor.

I doubt it ever actually occurred, but in theory at least, there was a
memory leak in set_tdesc_architecture and set_tdesc_osabi where the
member variables were assigned without freeing any previous
value... but I suspect that usually these fields are only set once.

There should be no user visible changes after this commit.

Approved-By: Tom Tromey <tom@tromey.com>
Approved-By: Simon Marchi <simon.marchi@efficios.com>