Fix compat32 on mips64

Summary:
Two bugs:

• Elf32_Auxinfo is broken, using pointers in the union, which are 64-bits not 32.
• freebsd32_sysarch() doesn't update the 'user local' register when handling MIPS_SET_TLS, leading to a NULL pointer dereference in the 32-bit application.

Reviewed by: MIPS, brooks
MFC after: 1 week
Sponsored by: Juniper Networks, Inc
Differential Revision: https://reviews.freebsd.org/D26556