mips: Save off the BSP HWREna register to share with APs

[jhibbits]

Summary

Without sharing the HWREna register it may default to something
undesirable on APs, leading to either reserved instruction exceptions or
emulated access, as in the UserLocal register (TCB pointer).

Sponsored by: Juniper Networks, Inc.

[adrian]

nice catch! please land on stable/13 and stable/12! tyvm!

[jrtc27]

sys/mips/mips/mp_machdep.c

63

This is already in cpuinfo

[jrtc27]

sys/mips/mips/mp_machdep.c

301

With #include <machine/cpuinfo.h>

[jrtc27]

Alternative proposal: https://reviews.freebsd.org/D48064

[jhibbits]

Abandoned in favor of D48064.