[PATCH, generic] Support printing of escaped curly braces and vertical bar in assembler output

[Maksim Kuznetsov]

> There are four in-tree target architectures that already use %|.  I think
> it would be better if you made these new escapes target-specific.

Escaped curly braces cannot be target-specific since
do_assembler_dialects() in final.c ignores any % and considers '{' and
'}' to be alternative delimeters.

> For the logic to find the end of an alternative, you can simply always
> skip over the next char after any percent sign (well, check for end of
> string, of course); there is no need to count percent signs.

That would not be a general approach. %% stands for printing percent
sign, so in assembler string "{%%}" closing curly brace should be
handled as the end of an alternative, though it follows a percent
sign.

-- 
Maxim Kuznetsov