Build both PIC and no PIC static libraries

When jemalloc is linked into an executable (as opposed to a shared
library), compiling with -fno-pic can have significant advantages,
mainly because we don't have to go throught the GOT (global offset
table).

Users who want to link jemalloc into a shared library that could
be dlopened need to link with libjemalloc_pic.a or libjemalloc.so.