gdb: make find_memory_region_ftype a function_view

gdb: make find_memory_region_ftype a function_view

Make it a function_view and remove the `void *` parameter.  Convert the
callers (in gcore.c) to use lambda functions that capture `obfd`.

The target_debug_print_find_memory_region_ftype debug printer isn't
terribly useful, but I don't see a good way to print a function_view
with what we have right now.

Since find_memory_region_ftype needs to be seen by both gdbarch.h and
target.h, I chose to move the typedef to a new file
(find-memory-region.h), that is included by both.

Change-Id: I9d24d31da31e5fe0439124cec1a9757ad226b222
Approved-By: Tom Tromey <tom@tromey.com>