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.