[scudo] Add a minimal runtime for -fsanitize-minimal-runtime compatibility

Summary:
This patch follows D48373.

The point is to be able to use Scudo with `-fsanitize-minimal-runtime`. For that
we need a runtime that doesn't embed the UBSan one. This results in binaries
that can be compiled with `-fsanitize=scudo,integer -fsanitize-minimal-runtime`.

Reviewers: eugenis

Reviewed By: eugenis

Subscribers: mgorny, delcypher, llvm-commits, #sanitizers

Differential Revision: https://reviews.llvm.org/D48377

llvm-svn: 335296