gdb: merge solib-frv aix-solib debug options into "set/show debug solib"

solib implementations are typically used one at a time.  So it will be
rare that you will want to enable debug for one solib kind, and
absolutely want to keep the others disabled.  To make things simpler,
instead of adding separate variables / macros / commands for each solib
implementation, merge the existing ones (frv and aix) into a unified
"set/show debug solib", with the solib_debug_printf macro.

Change-Id: I6e18bbc7401724f37ae66681badb079d75ecf7fa