Remove drti.o's dependency on libelf. This makes it possible to add DTrace
probes to userland programs and libraries without also needing to link
libelf.

dtrace -G places the SUNW_dof symbol at the beginning of the DOF (DTrace
probe and provider metdata) section in the generated object file; drti.o
now just uses this symbol to locate the section. A complication occurs
when multiple dtrace-generated object files are linked together, since the
SUNW_dof symbol defined in each file is global. This is handled by
using objcopy(1) to convert __SUNW_dof to a local symbol once drti.o has
been linked with the generated object file. Upstream, this is done using a
linker feature not present in GNU ld.

Differential Revision: https://reviews.freebsd.org/D1757
Reviewed by: rpaulo
MFC after: 1 month
Relnotes: yes