tslog: Annotate parts of SYSINIT cpu

Booting an amd64 kernel on Firecracker with 1 CPU and 128 MB of RAM,
SYSINIT cpu takes roughly 2770 us:

• 2280 us in vm_ksubmap_init
  • 535 us in kmem_malloc
    • 450 us in pmap_zero_page
  • 1720 us in pmap_growkernel
    • 1620 us in pmap_zero_page
• 80 us in bufinit
• 480 us in cpu_setregs
  • 430 us in cpu_setregs calling load_cr0

Much of this is hypervisor overhead: load_cr0 is slow because it traps
to the hypervisor, and 99% of the time in pmap_zero_page is spent when
we first touch the page, presumably due to the host Linux kernel
faulting in backing pages one by one.

Sponsored by: https://www.patreon.com/cperciva
Differential Revision: https://reviews.freebsd.org/D40327