[gn] Add target flags to asmflags.
[gn] Add target flags to asmflags.

Reviewers: pcc, thakis

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D58483

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@354618 91177308-0d34-0410-b5e6-96231b3b80d8

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