cgo_dynamic_linker directives to learn that the otherwise undefined
reference to sin in foo.cgo2.o should be rewritten to refer to the
symbol sin with version GLIBC_2.2.5 from the dynamic library
"libm.so.6", and the binary should request "/lib/ld-linux.so.2" as its
runtime dynamic linker.

In external mode, cmd/link does not process any host object files, in
particular foo.cgo2.o. It links together the gc-generated object