[RISCV] Move architecture parsing code into its own function
[RISCV] Move architecture parsing code into its own function

I plan to reuse it in a later patch.

This is almost NFC except a small change in control flow when diagnosing
+d without +f.

Differential Revision: https://reviews.llvm.org/D66002

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@371492 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang