[scudo] Soft and hard RSS limit checks

Summary:
This implements an opportunistic check for the RSS limit.

For ASan, this was implemented thanks to a background thread checking the
current RSS vs the set limit every 100ms. This was deemed problematic for Scudo
due to potential Android concerns (Zygote as pointed out by Aleksey) as well as
the general inconvenience of having a permanent background thread.

If a limit (soft or hard) is specified, we will attempt to update the RSS limit
status (exceeded or not) every 100ms. This is done in an opportunistic way: if
we can update it, we do it, if not we return the current status, mostly because
we don't need it to be fully consistent (it's done every 100ms anyway). If the
limit is exceeded `allocate` will act as if OOM for a soft limit, or just die
for a hard limit.

We use the `common_flags()`'s `hard_rss_limit_mb` & `soft_rss_limit_mb` for
configuration of the limits.

Reviewers: alekseyshl

Reviewed By: alekseyshl

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D40038

llvm-svn: 318301

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