[hwasan] Allow optional early shadow setup.

Summary:
Provide __hwasan_shadow_init that can be used to initialize shadow w/o touching libc.
It can be used to bootstrap an unusual case of fully-static executable with
hwasan-instrumented libc, which needs to run hwasan code before it is ready to serve
user calls like madvise().

Reviewers: vitalybuka, kcc

Subscribers: kubamracek, llvm-commits

Differential Revision: https://reviews.llvm.org/D50581

llvm-svn: 339606