X86AsmPrinter MCJIT MSVC bug fix.

Summary:
This bug was introduced in r213006 which makes an assumption that MCSection is COFF for Windows MSVC. This assumption is broken for MCJIT users where ELF is used instead [1]. The fix is to change the MCSection cast to a dyn_cast.

[1] http://lists.cs.uiuc.edu/pipermail/llvmdev/2013-December/068407.html.

Reviewers: majnemer

Reviewed By: majnemer

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D4872