[ReleaseNotes] Add note for enabling GlobalISel for AArch64 -O0
[ReleaseNotes] Add note for enabling GlobalISel for AArch64 -O0

Patch by Amara Emerson.

Differential Revision: https://reviews.llvm.org/D42861


git-svn-id: aad6896a0674cbe2af7806761e473047ca8ef14a@324211 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm