Switch to use the cxx.link_style to define the default

Summary:
Configure the LinkerInfo in toolchain impl via attr.

#buildmore

Reviewed By: milend

Differential Revision: D51861188

fbshipit-source-id: 3f09350332954aa652476315f59a8a67b54dbe94

prelude/cxx/cxx_toolchain.bzl

@@ -83,7 +83,7 @@ def cxx_toolchain_impl(ctx):
         is_pdb_generated = is_pdb_generated(ctx.attrs.linker_type, ctx.attrs.linker_flags),
         link_binaries_locally = not value_or(ctx.attrs.cache_links, True),
         link_libraries_locally = False,
-        link_style = LinkStyle("static"),
+        link_style = LinkStyle(ctx.attrs.link_style),
         link_weight = 1,
         link_ordering = ctx.attrs.link_ordering,
         linker = ctx.attrs.linker[RunInfo],

prelude/decls/cxx_rules.bzl

@@ -906,6 +906,7 @@ cxx_toolchain = prelude_rule(
             "labels": attrs.list(attrs.string(), default = []),
             "licenses": attrs.list(attrs.source(), default = []),
             "link_path_normalization_args_enabled": attrs.bool(default = False),
+            "link_style": attrs.string(default = "static"),
             "linker": attrs.source(),
             "linker_flags": attrs.list(attrs.arg(anon_target_compatible = True), default = []),
             "linker_type": attrs.enum(LinkerProviderType),