[lldb] [gdb-remote] Try using <architecture/> for remote arch unconditionally

Try determining the process architecture from <architecture/> tag
unconditionally, rather than for very specific cases.  Generic gdbserver
implementations do not support LLDB-specific packets used to determine
the process architecture, therefore this fallback is necessary to
support architecture-specific behavior on these targets.  Rather than
maintaining a mapping of all known architectures, just try mapping
the GDB values into triplets, as that is going to work most of the time.

This change is confirmed to fix LLDB against gdbserver when debugging
i386 and aarch64 executables.

Differential Revision: https://reviews.llvm.org/D109272