Fix-validation kernel build (guest /usr/src, patched sys/bus/gpio/gpio_acpi/gpio_acpi.c):

$ cd /usr/src && make -j6 nativekernel KERNCONF=X86_64_GENERIC && make installkernel KERNCONF=X86_64_GENERIC
...
>>> Kernel install for X86_64_GENERIC completed on Tue Oct  6 19:57:40 UTC 2026
--------------------------------------------------------------
RC=0

(First two attempts failed to compile: atomic_sub_int -> DF x86_64 spells it
 atomic_subtract_int (sys/cpu/x86_64/include/atomic.h); final fix.diff uses
 atomic_subtract_int and adds <machine/atomic.h>. Build log excerpt above is
 from the guest /tmp/kbuild3.log, quoted here because the guest is reset to
 the clean-source snapshot after validation.)