Bug#914770: llvm-toolchain-7: baseline violation on i386

[Fanael Linithien]

I fail to see how #914770 and #914838 are related in any way at all,
the former blockers of #894840 and #632472 are much more relevant in
my opinion, even if still not ideal.

Anyway, it's still a baseline violation that affects *binary packages
already in sid*, in my case breaking seemingly unrelated packages like
mesa, and due to the recent change of how LLVM is built (i.e.
bootstrapped using GCC for stage 1, then Clang for stage 2), it
affects *all packages* built from src:llvm-toolchain-7 and thus should
be reported against that.

Furthermore, I don't think changing the baseline would be easy,
because of Firefox: stable uses the latest ESR, which in turn depends
on recent Rust, which in turn depends on recent LLVM, so in one or two
Firefox ESR cycles we (Debian) may end up requiring LLVM 7 in *jessie
and stretch*, and while changing the baseline of buster may be an
option[1], changing it retroactively in *oldstable and stable* doesn't
seem like a very good idea.

At any rate, the fix to this particular issue seems quite easy: just
pass -march=i686 to Clang when building stage 2 of LLVM and Clang.

[1] And even then, I'd be willing to contribute to keep the baseline
as is for as long as possible: I have the old hardware to test stuff
on, I have the knowledge to fix stuff when needed, and I have the
willingness to make Debian better.