Set CP15BEN in SCTLR to make memory barriers work in 32-bit mode.

Binaries generated by Clang for ARMv6 may contain these instructions:

MCR p15, 0, <Rd>, c7, c10, 5

These instructions are deprecated as of ARMv7, which is why modern
processors have a way of toggling support for them. On FreeBSD/arm64 we
currently disable support for these instructions, meaning that if 32-bit
executables with these instructions are run, they would crash with
SIGILL. This is likely not what we want.

Reviewed by: andrew
Differential Revision: https://reviews.freebsd.org/D13145