Remove unused basemem calculation

[emaste]

Summary

It seems basemem isn't used in amd64 startup at all.

UEFI may not put a usable memory range at physaddr 0 which leads to a panic when basemem == 0, but the AP startup trampoline is put in a hole carved out of the first usable segment and it seems like it should be fine regardless of whether that starts at physaddr 0 or not.

We'll still have problems if the first range is too small, or starts at a physaddr address too high, but we would have panicked in that case anyway. I can revisit adding an explicit check for that later.

[emaste]

kib pointed out that mptable.c needs basemem; it's not in GENERIC but may be used in some cases.

[jhb]

basemem is implicitly rooted at zero, so you can't just pick any segment with memory below 1M, though you could possibly cheat that way. You could probably rewrite mptable to use physmap[] directly and look for its table in segements that match the specs requirements.