This copies logic from amd64 and RISC-V. Now, upon boot my espressobin
prints:
real memory = 1073737728 (1023 MB) avail memory = 1025900544 (978 MB)
A follow-up patch to turn this into an MI subroutine would be nice. It
is easier than it used to be now that phys_avail[] is an MI structure.
PR: 243682