This reverts e08c946c and 05165c2f. The actual intention of
e08c946c was to fix shared library builds for arm/win32, which
can also be accomplished in other ways.

Disabling pic on those platforms broke inline assembly on cygwin/64
(since some inline assembly requires knowing whether we are building
as PIC or not), and might also break inline assembly on other
compilers on windows.

As a side-effect, this unfortunately brings back all the warnings
about PIC not having any effect on that platform.
Signed-off-by: Martin Storsjö <martin@martin.st>