Bug#914268: llvm-toolchain-7: baseline violation on armhf

[Adrian Bunk]

Source: llvm-toolchain-7
Version: 1:7.0.1~+rc2-1~exp1
Severity: grave
Control: block 912164 by -1
Control: affects -1 src:rust-pulldown-cmark src:mesa src:cargo

LLVM now violates the armhf baseline by using NEON,
this can be reproduced as easy as:

(sid_armhf-dchroot)bunk at abel:~$ llvm-config-7
Illegal instruction
(sid_armhf-dchroot)bunk at abel:~$

gdb confirms that this is due to NEON usage.


Additionally, this also causes FTBFS on some of our buildds:
https://buildd.debian.org/status/fetch.php?pkg=rust-pulldown-cmark&arch=armhf&ver=0.2.0-1&stamp=1542734241&raw=0
https://buildd.debian.org/status/fetch.php?pkg=cargo&arch=armhf&ver=0.31.0-4&stamp=1542700136&raw=0
https://buildd.debian.org/status/fetch.php?pkg=mesa&arch=armhf&ver=18.2.5-2&stamp=1542760450&raw=0

It might even result in silently misbuilt packages, just imagine
mesa would have disabled the feature instead of AC_MSG_ERROR.


Note that these mentioned buildd problems are just the visible
tip of the iceberg, the baseline violation is a problem that
must be fixed even when the buildds change.