[SanitizerCoverage] Optimize stack-depth instrumentation.

Summary:
Use the initialexec TLS type and eliminate calls to the TLS
wrapper.  Fixes the sanitizer-x86_64-linux-fuzzer bot failure.

Reviewers: vitalybuka, kcc

Reviewed By: kcc

Subscribers: hiraditya, llvm-commits

Differential Revision: https://reviews.llvm.org/D37026

llvm-svn: 311490