Teach GDB to generate sparse core files (PR corefiles/31494)

This commit teaches GDB's gcore command to generate sparse core files
(if supported by the filesystem).

To create a sparse file, all you have to do is skip writing zeros to
the file, instead lseek'ing-ahead over them.

The sparse logic is applied when writing the memory sections, as
that's where the bulk of the data and the zeros are.

The commit also tweaks gdb.base/bigcore.exp to make it exercise
gdb-generated cores in addition to kernel-generated cores.  We
couldn't do that before, because GDB's gcore on that test's program
would generate a multi-GB non-sparse core (16GB on my system).

After this commit, gdb.base/bigcore.exp generates, when testing with
GDB's gcore, a much smaller core file, roughly in line with what the
kernel produces:

 real sizes:

 $ du --hu testsuite/outputs/gdb.base/bigcore/bigcore.corefile.*
 2.2M    testsuite/outputs/gdb.base/bigcore/bigcore.corefile.gdb
 2.0M    testsuite/outputs/gdb.base/bigcore/bigcore.corefile.kernel

 apparent sizes:

 $ du --hu --apparent-size testsuite/outputs/gdb.base/bigcore/bigcore.corefile.*
 16G     testsuite/outputs/gdb.base/bigcore/bigcore.corefile.gdb
 16G     testsuite/outputs/gdb.base/bigcore/bigcore.corefile.kernel

Time to generate the core also goes down significantly.  On my machine, I get:

  when writing to an SSD, from 21.0s, down to 8.0s
  when writing to an HDD, from 31.0s, down to 8.5s

The changes to gdb.base/bigcore.exp are smaller than they look at
first sight.  It's basically mostly refactoring -- moving most of the
code to a new procedure which takes as argument who should dump the
core, and then calling the procedure twice.  I purposely did not
modernize any of the refactored code in this patch.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31494


Reviewed-By: Lancelot Six <lancelot.six@amd.com>
Reviewed-By: Eli Zaretskii <eliz@gnu.org>
Reviewed-By: John Baldwin <jhb@FreeBSD.org>
Change-Id: I2554a6a4a72d8c199ce31f176e0ead0c0c76cff1