[extract_symbols.py] Fix llvm-readobj options.

As of D105532, a couple of llvm-readobj options have slightly changed
their spellings. '-file-headers' has become '--file-header', and
'-symbols' has become '--symbols'. A modern llvm-readobj will respond
to those single-dash spellings by trying to parse them as combinations
of single-letter options, so that you get "unknown argument '-f'" for
-file-headers, and "unknown argument '-y'" for the second letter of
-symbols.

extract_symbols.py was still invoking llvm-readobj with those old
single-dash spellings. Changed them to the newly canonical ones, which
as far as I can tell still work on llvm-readobj from before the
change.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D113556