Only one linker_flags attribute

Summary:
This attribute was present in both `rust_common_attributes` and `rust_common.linker_flags`. `rust_binary` happened to have it added to its attribute dict twice.

Refactor so that we only define the attribute once. The particular reason for choosing this refactor is the next diff

Reviewed By: IanChilds

Differential Revision: D49004164

fbshipit-source-id: 64b682a80541ffff27f0ee62142a3ea5c7abece8

Author: JakobDegen

prelude/decls/rust_common.bzl

@@ -65,7 +65,7 @@ def _rustc_flags_arg():
 
 def _linker_flags_arg():
     return {
-        "linker_flags": attrs.list(attrs.arg(), default = [], doc = """
+        "linker_flags": attrs.list(attrs.arg(anon_target_compatible = True), default = [], doc = """
     The set of additional flags to pass to the linker.
 """),
     }

prelude/decls/rust_rules.bzl

@@ -77,13 +77,6 @@ def _rust_common_attributes(is_binary: bool):
         "incremental_enabled": attrs.bool(default = False),
         "labels": attrs.list(attrs.string(), default = []),
         "licenses": attrs.list(attrs.source(), default = []),
-        # linker_flags weren't supported for rust_library in Buck v1 but the
-        # fbcode macros pass them anyway. They're typically empty since the
-        # config-level flags don't get injected, but it doesn't hurt to accept
-        # them and it simplifies the implementation of Rust rules since they
-        # don't have to know whether we're building a rust_binary or a
-        # rust_library.
-        "linker_flags": attrs.list(attrs.arg(anon_target_compatible = True), default = []),
         "resources": attrs.named_set(attrs.one_of(attrs.dep(), attrs.source()), sorted = True, default = []),
         "rustdoc_flags": attrs.list(attrs.arg(), default = []),
         "version_universe": attrs.option(attrs.string(), default = None),
@@ -231,6 +224,13 @@ rust_library = prelude_rule(
         rust_common.edition_arg() |
         rust_common.features_arg() |
         rust_common.rustc_flags_arg() |
+        # linker_flags weren't supported for rust_library in Buck v1 but the
+        # fbcode macros pass them anyway. They're typically empty since the
+        # config-level flags don't get injected, but it doesn't hurt to accept
+        # them and it simplifies the implementation of Rust rules since they
+        # don't have to know whether we're building a rust_binary or a
+        # rust_library.
+        rust_common.linker_flags_arg() |
         rust_common.env_arg() |
         rust_common.crate(crate_type = attrs.option(attrs.string(), default = None)) |
         rust_common.crate_root() |
@@ -318,6 +318,7 @@ rust_test = prelude_rule(
         rust_common.edition_arg() |
         rust_common.features_arg() |
         rust_common.rustc_flags_arg() |
+        rust_common.linker_flags_arg() |
         rust_common.env_arg() |
         rust_common.crate(crate_type = attrs.option(attrs.string(), default = None)) |
         rust_common.crate_root() |