[AArch64] Use '//' as comment string for MSVC assembly

As the actual MSVC toolset doesn't use the GAS-style assembly that
Clang/LLVM produces and consumes, there's no reference for what
string to use for e.g. comments when building with a MSVC triple.

This frees up the use of semicolon as separator string, just like
was done for GNU targets in 23413195.
(Previously, both the separator and comment strings were set to
the same, a semicolon.)

Compiler-rt extensively uses separator chars in its assembly,
and that assembly should be buildable with clang-cl for MSVC too.

Differential Revision: https://reviews.llvm.org/D96259

(cherry picked from commit 71c29b4c)