HWASan support in bionic.

* Allow sanitization of libc (excluding existing global sanitizers)
  and disallow sanitization of linker. The latter has not been
  necessary before because HWASan is the first sanitizer to support
  static binaries (with the exception of CFI, which is not used
  globally).
* Static binary startup: initialize HWASan shadow very early so that
  almost entire libc can be sanitized. The rest of initialization is
  done in a global constructor; until that is done sanitized code can
  run but can't report errors (will simply crash with SIGTRAP).
* Switch malloc_common from je_*  to __sanitizer_*.
* Call hwasan functions when entering and leaving threads. We can not
  intercept pthread_create when libc depends on libclang_rt.hwasan.
  An alternative to this would be a callback interface like requested
  here:
    https://sourceware.org/glibc/wiki/ThreadPropertiesAPI

All of the above is behind a compile-time check
__has_feature(hwaddress_sanitizer). This means that HWASan actually
requires libc to be instrumented, and would not work otherwise. It's
an implementation choice that greatly reduces complexity of the tool.
Instrumented libc also guarantees that hwasan is present and
initialized in every process, which allows piecemeal sanitization
(i.e. library w/o main executable, or even individual static
libraries), unlike ASan.

Change-Id: If44c46b79b15049d1745ba46ec910ae4f355d19c

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