Move renaming of local symbols after checking for branch instructions

This simplifies identifying local symbols within the branch
heuristic block.
Add an option for trying to automatically add a prefix to external symbols
Move the initial handling of lines into parse_line
Comment out gnu specific stack notes

Libav doesn't output these at all unless we're targeting ELF.
Add initial support for .include

This misses support for checking include directories specified
with -I parameters.
Don't comment out the whole line when handling .type

libopus piles up multiple directives on one line.
Only interpret .if if it's the first word of a line

The arm2gnu.pl script in libopus converts an IF (from a license
header) into a .if, which messed up gas-pp.