[asan] Initialize fake stack during thread init

If detect-stack-use-after-return is on, initialize fake stack during
AsanThread::Init(), rather than lazily.  This is required on Myriad.
From kcc: "There used to be a reason why this was done lazily, but I
don't remember if we still have that reason."  Tested on x86.

Differential Revision: https://reviews.llvm.org/D46626

llvm-svn: 332033