[WebAssembly] Compute and export TLS block alignment
Add immutable WASM global __tls_align which stores the alignment
requirements of the TLS segment.
Add __builtin_wasm_tls_align() intrinsic to get this alignment in Clang.
The expected usage has now changed to:
Reviewers: tlively, aheejin, sbc100, sunfish, alexcrichton
Reviewed By: tlively
Subscribers: dschuff, jgravelle-google, hiraditya, cfe-commits, llvm-commits
Tags: #clang, #llvm
Differential Revision: https://reviews.llvm.org/D65028