jemalloc: don't expose 3.0 compat symbols

Don't provide default linkage for jemalloc 3.0 compatability symbols.
We stopped declaring these interfaces with the introduction of jemalloc
4.0 prior to FreeBSD 11.0. Any code using them would have had to
declare them manually so stop declaring them and export the symbols
directly for compatability. Arguably they should be x86 only as they
were never declared on other Tier-1 architectures.

Reviewed by: imp, kib
Differential Revision: https://reviews.freebsd.org/D46407