cmd/link: handle dynamic import variables on Darwin
Currently, on darwin, we only support cgo_dynamic_import for
functions, but not variables, as we don't need it before.
mach_task_self_ is a variable defined in the system library, which
can be used to e.g. access the process's memory mappings via the
mach API. The C header defines a macro mach_task_self(), which
refers to the variable. To use mach_task_self_ (in pure-Go
programs) we need to access it in Go.
This CL handles cgo_dynamic_import for variables in the linker,
loading its address via the GOT. (Currently only on Darwin, as
we only need it there.)
For #50891.
Change-Id: Idf64fa88ba2f2381443a1ed0b42b14b581843493
Reviewed-on: https://go-review.googlesource.com/c/go/+/501855
Run-TryBot: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org> Reviewed-by: Than McIntosh <thanm@google.com>