[GWP-ASan] Integrate GWP-ASan into bionc's malloc() (using hooks).

This patch introduces GWP-ASan - a sampled allocator framework that
finds use-after-free and heap-buffer-overflow bugs in production
environments.

GWP-ASan is being introduced in an always-disabled mode. This means that
GWP-ASan will be permanently disabled until a further patch turns on
support. As such, there should be no visible functional change for the
time being.

GWP-ASan requires -fno-emulated-tls wherever it's linked from. We
intentionally link GWP-ASan into libc so that it's part of the initial
set of libraries, and thus has static TLS storage (so we can use
Initial-Exec TLS instead of Global-Dynamic). As a benefit, this reduces
overhead for a sampled process.

GWP-ASan is always initialised via. a call to
mallopt(M_INITIALIZE_GWP_ASAN, which must be done before a process is
multithreaded).

More information about GWP-ASan can be found in the upstream
documentation: http://llvm.org/docs/GwpAsan.html

Bug: 135634846
Test: atest bionic
Change-Id: Ib9bd33337d17dab39ac32f4536bff71bd23498b0

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