gdb/solib-frv: remove manual memory management

 - Make fetch_loadmap return a unique pointer, adjust callers
 - Make fetch_loadmap use a unique pointer for ext_ldmbuf
 - Replace some fields of lm_info_frv to be unique pointers
 - Make main_executable_lm_info a unique pointer

I can only build-test this.

Change-Id: I5782ea31b5e30fef2c7468f0bc24d10ffae92669
Approved-by: Kevin Buettner <kevinb@redhat.com>