MC: Change MCAsmBackend::writeNopData() to take a raw_ostream instead of an MCObjectW...

MC: Change MCAsmBackend::writeNopData() to take a raw_ostream instead of an MCObjectWriter. NFCI.

To make this work I needed to add an endianness field to MCAsmBackend
so that writeNopData() implementations know which endianness to use.

Part of PR37466.

Differential Revision: https://reviews.llvm.org/D47035

llvm-svn: 332857