buck: make objcopy available via cxx_genrule

Summary:
following advice in:

https://fb.workplace.com/groups/930797200910874/permalink/1352691642054759/

this diff makes it possible to use a reference to the toolchain's objcopy via, e.g.

```
cxx_genrule(
    name = "embed_default_sensor_data",
    srcs = [
        "sensor_data",
        "embed.sh",
    ],
    outs = {
        "object": ["default_sensor_data_lib"],
    },
    bash = "./embed.sh $(ld) $(objcopy)",
    visibility = ["PUBLIC"],
)
```

Reviewed By: dtolnay

Differential Revision: D52817209

fbshipit-source-id: 92ae52a595a185641310c8c1fe200fdaa1dcc54b

prelude/cxx/cxx_toolchain_types.bzl

@@ -306,6 +306,7 @@ def cxx_toolchain_infos(
         "ldflags-shared": _shell_quote(linker_info.linker_flags),
         "ldflags-static": _shell_quote(linker_info.linker_flags),
         "ldflags-static-pic": _shell_quote(linker_info.linker_flags),
+        "objcopy": binary_utilities_info.objcopy,
         # TODO(T110378148): $(platform-name) is almost unusued. Should we remove it?
         "platform-name": platform_name,
     }