Bug#864847: ghc: on armhf should not use deprecated memory barrier instructions

[Edmund Grimley Evans]

Source: ghc
Version: 8.0.1-17+b1

If I try to run "ghc" in one of my armhf chroots, it does not work very well:

$ ghc
Illegal instruction

The offending instruction is this one:

mcr     15, 0, r6, cr7, cr10, {5}

This is, I think, an ARMv6 memory barrier, and these instructions are,
if I recall correctly, deprecated in ARMv7 and removed entirely in
ARMv8, though the kernel can be configured to emulate them. Since
armhf requires at least ARMv7, GHC should not be using or generating
these instructions on armhf.

I presume this problem concerns all binaries generated by GHC.

However, I hope it does not prevent any armhf binary from running
under a Debian armhf stock kernel image, even on ARMv8 hardware,
though someone should perhaps check that last point and raise the
severity if it turns out there is a problem on any standard
installation.