[mips] Switch to the `.text` section after emitting asm file preamble

Now the last `.section` directive in the MIPS asm file preamble
is the `.section .mdebug.abi`. If assembler code injected for example
by the LLVM `module asm` or the C ` __asm` directives do not contain
explicit switching to the `.text` section it goes to the `.mdebug.abi`
section. It might be unexpected to the user and in fact for example
breaks building some existing code like FreeBSD libc [1].

The patch forces switching to the `.text` section after emitting MIPS
assembler file preamble.

[1] https://bugs.llvm.org/show_bug.cgi?id=43119

Fix PR43119.

Differential Revision: https://reviews.llvm.org/D67014

llvm-svn: 370735