amd64: do not touch BIOS reset flag halfword, unless we boot through BIOS

[markj]

sys/amd64/amd64/machdep.c

1183

I think it would be better to explicitly cast the address, i.e., (unsigned short *)0x472.

It seems like a strange place to do this operation. hammer_time() already checks for MODINFOMD_EFI_MAP to see if vty_set_preferred() should be called; we could do the same check earlier in order to set the BIOS flag, and perhaps save the result to avoid a second preload_search_info() call.

[kib]

Move the write to hammer_time()