[CodeGen] Hoist common AsmPrinter code out of X86, ARM, and AArch64

[CodeGen] Hoist common AsmPrinter code out of X86, ARM, and AArch64

Every known PE COFF target emits /EXPORT: linker flags into a .drective
section. The AsmPrinter should handle this.

While we're at it, use global_values() and emit each export flag with
its own .ascii directive. This should make the .s file output more
readable.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@322788 91177308-0d34-0410-b5e6-96231b3b80d8