Always print the implicit .text at the start of an asm file.

Before llvm-mc would print it, but llc was assuming that it would produce
another section changing directive before one was needed. That assumption is
false with inline asm.

Fixes PR19049.

Another option would be to always create the section, but in the asm printer
avoid printing sections changes during initialization. That would work, but
* We do use the fact that llvm-mc prints it in testing. The tests can be changed
  if needed.
* A quick poll on IRC suggest that most developers prefer the implicit .text to
  be printed.