[extract_symbols.py] Fix line-splitting of tool output.

Two functions in the `is_32bit_windows` family were retrieving the
output of a tool via `subprocess.check_output`, and then iterating
over it using `for line in output`. But in Python, that gets you the
output one //character// at a time, not a line at a time. So the
regexes that looked for a platform name were never matching.

(This is a mistake that Python makes uniquely easy, because iterating
over a file and over a string have different default behaviour, and
because the element type of a string is still a string so you don't
even get a type mismatch error to warn you about it!)

Reviewed By: michaelplatings

Differential Revision: https://reviews.llvm.org/D117030