[flang] Run pic-flags.f90 test in more places

83a06997 broke this test but
we only saw the failure on a subset of the flang bots because
it requires 3 target backends, and most of ours only enable AArch64.

Use %if to run as much of the test as we can with the targets
we have.

Reviewed By: awarzynski

Differential Revision: https://reviews.llvm.org/D158264