[lsan] Enable LSan for x86 Linux.

People keep asking LSan to be available on 32 bit targets (e.g. https://github.com/google/sanitizers/issues/403)
despite the fact that false negative ratio might be huge (up to 85%). This happens for big real world applications
that may contain random binary data (e.g. browser), but for smaller apps situation is not so terrible and LSan still might be useful.
This patch adds initial support for x86 Linux (disabled by default), ARM32 is in TODO list.
We used this patch (well, ported to GCC) on our 32 bit mobile emulators and it worked pretty fine
thus I'm posting it here to initiate further discussion.

Differential Revision: https://reviews.llvm.org/D28609

llvm-svn: 292775