gdbserver: move main_lm handling into caller

gdbserver: move main_lm handling into caller

When listing SVR4 shared libraries, special care has to be taken about the
first library in the default namespace as that refers to the main
executable.  The load map address of this main executable is provided in
an attribute of the library-list-svr4 element.

Move that code from where we enumerate libraries inside a single namespace
to where we generate the rest of the library-list-svr4 element.  This
allows us to complete the library-list-svr4 element inside one function.

There should be no functional change.