This change adds the global physHugePageSize which is initialized in
osinit(). physHugePageSize contains the system's transparent huge page
(or superpage) size in bytes.

For #30333.

Change-Id: I2f0198c40729dbbe6e6f2676cef1d57dd107562c
Reviewed-on: https://go-review.googlesource.com/c/go/+/170858
Run-TryBot: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Austin Clements <austin@google.com>