gdb: make structured core file mappings processing global

gdb: make structured core file mappings processing global

In corelow.c, within core_target::build_file_mappings, we have code
that wraps around a call to gdbarch_read_core_file_mappings and
provides more structure to the results.

Specifically, gdbarch_read_core_file_mappings calls a callback once
for every region of every mapped file.  The wrapper code groups all of
the mappings for one file into an instance of 'struct mapped_file',
this allows all of the mapped regions to be associated with the
build-id and filename of a file.

In the next commit I plan to make this information available via the
Python API, and so I need to allow access to this structured wrapping
outside of corelow.c.

This commit renames 'struct mapped_file' to 'struct core_mapped_file'
and moves the struct into gdbcore.h.  Then a new global function
gdb_read_core_file_mappings is created into which I move the code to
build the structured data.

Then corelow.c is updated to call gdb_read_core_file_mappings.

This commit does not extend the Python API, that is for the next
commit.

There should be no user visible changes after this commit.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=32844

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