[lldb] Only send "posix" error codes through the gdb-remote protocol (llvm#108170)

The other side has no way of telling which namespace do these codes
belong to, so mashing them all together is not very helpful.

I'm mainly doing this to simplify some code in a pending patch
<https://github.com/llvm/llvm-project/pull/106774/files#r1752628604>,
and I've picked the posix error category semi-randomly. If we wanted to
be serious about assigning meaning to these error codes, we should
create a special error category for "gdb errors".

Author: labath