arch/posix: Fix 32 bit x86 fuzzing

It seems like libfuzzer wants to relocate 32 bit instrumented code
sections at runtime at addresses different than the ones in the ELF
file.  This is problematic, because Zephyr files are compiled
statically and so will crash the first time they try to jump to an
absolute .text address (basically at the first function call after a
fuzzer entry point).

It seems that building with -fPIC is enough to defeat this (we use the
host linker script, which will manage the GOT/PLT entries for us),
which will work as long as the fuzzer isn't playing games with data
other than text.  None of this seems to be documented, so... I guess
it's as good as we can get.  It works, at least.

(x86_64 binaries don't show the same behavior, they run where they
were linked)

Signed-off-by: Andy Ross <andyross@google.com>