libdtrace: Change the binding of USDT probe symbols to STB_WEAK

Otherwise, if multiple object files contain references to the same
probe, newish lld will refuse to link them by default, raising a
duplicate global symbol definition error. Previously, duplicate global
symbols with identical absolute st_values were permitted by both lld and
GNU ld.

Since dtrace has no use for probe function symbols after the relocation
performed by dtrace -G, make the symbols weak as well, following a
suggestion from MaskRay.

Reported by: dim
MFC after: 1 week
Sponsored by: The FreeBSD Foundation