dwarf_attrval_signed(3) on FreeBSD

The text eb2cd8a985fd, as FreeBSD 12.0-RELEASE, FreeBSD 11.4-RELEASE, FreeBSD 11.3-RELEASE, FreeBSD 11.2-RELEASE, FreeBSD 11.1-RELEASE shipped it.

retrieve the value of an attribute within a DWARF debugging information entry