[WebAssembly] Change comments on SP writing back (NFC)

Summary: We now write back not to memory but to __stack_pointer global.

Reviewers: dschuff

Subscribers: sbc100, jgravelle-google, sunfish, llvm-commits

Differential Revision: https://reviews.llvm.org/D51074

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@340372 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed