lld-link: align sections to 16 bytes if referenced from the gfids table

Summary:
Control flow guard works best when targets it checks are 16-byte aligned.
Microsoft's link.exe helps ensure this by aligning code from sections
that are referenced from the gfids table to 16 bytes when linking with
-guard:cf, even if the original section specifies a smaller alignment.
This change implements that behavior in lld-link.

See https://crbug.com/857012 for more details.

Reviewers: ruiu, hans, thakis, zturner

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D48690

llvm-svn: 335864