[MC] Allow Stackmap sections after DWARF in MachO.

Summary:
Mixing stackmaps and DWARF in a single file triggers an assertion in
MCMachOStreamer as stackmap sections are emitted in AsmPrinter::emitEndOfAsmFile
after the DWARF sections have already been emitted.

Since it should be safe to emit stackmap sections after DWARF sections this
patch relaxes the assertion to allow that.

Reviewers: aprantl, dblaikie, echristo

Subscribers: hiraditya, ributzka, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D75836