libc: add LIBC_MALLOC option

This will enable alternative mallocs to be included in the tree and
selected by setting LIBC_MALLOC. As there is only one today (jemalloc)
this option does nothing, but we expect to add other implementations
in the future. This will also reduce diffs to CheriBSD.

Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D41660