sysutils/beats*: fix build on armv7

The patch "patch-go-sysinfo" ports a Linux-only go module to FreeBSD.
Due to two oversights, the patch did not build on armv7. Fix the patch,
ensuring that all beats versions build on armv7 FreeBSD. A subtle bug
is addressed, too: the patch would previously convert from microseconds
to nanoseconds for time.Unix() by multiplying with time.Microsecond,
which is not the correct value. Multiply with 1000 instead.

Approved by: portmgr (build fix blanket)
MFH: 2026Q2