helper/log: Fix build using _DEBUG_FREE_SPACE_

helper/log: Fix build using _DEBUG_FREE_SPACE_

The glibc API 'mallinfo' is deprecated and the new 'mallinfo2'
should be used from glibc 2.33 (2021-02-01).

Throw an error when '--enable-malloc-logging' is used on systems
that compile without glibc.
Detect the glibc version and, for backward compatibility, define
'mallinfo2' as the old 'mallinfo'. Define a macro for the format
of 'fordblks'.

Change-Id: I68bff7b1b58f0ec2669db0b911f19c1c5a26ed30
Reported-by: Steven J. Hill <steven.hill@konsulko.com>
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/8589
Tested-by: jenkins