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

[Fanael Linithien]

I've verified that adding the following to the llvm-toolchain-7's
debian/rules and rebuilding fixes the issue:

ifneq (,$(filter $(DEB_HOST_ARCH),i386))
# Clang default to baseline-violating -march setting on i386.
CFLAGS_EXTRA += -march=i686
CXXFLAGS_EXTRA += -march=i686
endif