[mem]: fix OpenBSD virtual memory on -current

VirtualMemory() on OpenBSD reads the vfs.generic.bcachestat sysctl into
the generated Bcachestats struct and rejects the result when the kernel
returns fewer bytes than sizeOfBcachestats:

    error getting virtual memory info: short syscall ret 104 bytes

OpenBSD removed the buffer cache "flipper" in kernel commit ff03df9b11f
("Remove the buffer flipper", 2026-06-10), which deletes the dmapages,
highpages, highflips, highflops and dmaflips fields from struct
bcachestats (sys/sys/mount.h). struct bcachestats is now 13 int64_t
fields = 104 bytes on every architecture.

https://marc.info/?l=openbsd-cvs&m=178104984860610&w=2

Our generated per-arch files still expected the larger struct (0x78 on
amd64, 0x90 on the others), so the length guard tripped and mem metrics
broke on -current.

Regenerate the struct for all OpenBSD arches: drop the removed fields and
set sizeOfBcachestats to 0x68 (104). Only Numbufpages (offset 8) is
consumed, and the guard is `length < sizeOfBcachestats`, so this stays
compatible with older OpenBSD releases that return a larger struct.