Add workaround for jemalloc linking order

Add workaround for jemalloc linking order

Because we don't use jemalloc functions directly, but only via the
libisc library, the dynamic linker might pull the jemalloc library
too late when memory has been already allocated via standard libc
allocator.

Add a workaround round isc_mem_create() that makes the dynamic linker
to pull jemalloc earlier than libc.