pass -fPIC to the assembler, not linker

-fPIC has no effect on linking although it seems to be ignored by
GNU ld.bfd. However, it causes ld.lld to terminate with an invalid
argument error.

Reviewed by: dchagin, kib
MFC after: 1 month
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D5444