Disable ASAN for Fiber::init()

Disable ASAN for Fiber::init()

Summary:
Fiber::init() method modifies all the bytes in the fiber's stack in
order to produce a high watermark of stack memory usage. This behavior
generates ASAN errors, as it modifies stack red zones. For that reason,
ASAN is disabled for that particular method.

Test Plan:
Unit tests

Reviewed By: alikhtarov@fb.com

Subscribers: folly-diffs@, yfeldblum, chalfant

FB internal diff: D2036280

Tasks: 6861962

Signature: t1:2036280:1430439550:e156e0e06fae3615d9d9204e99acaf3072bbfafd