rust: Use host linker when building for Mac host.

This behavior is non-hermetic, but matches the behavior of the C++
support. On Linux, using lld works, but on OSX, this fails because lld
does not support Apple's new .tbd files.

Bug: 155302034
Test: On both Linux and Mac hosts:
    cd external/rust; mma

Change-Id: I0ad489113d720bdb9c3b7a67cce9d1e72266f428

rust/config/x86_darwin_host.go

@@ -21,8 +21,10 @@ import (
 )
 
 var (
-	DarwinRustFlags      = []string{}
-	DarwinRustLinkFlags  = []string{}
+	DarwinRustFlags     = []string{}
+	DarwinRustLinkFlags = []string{
+		"-B${ccConfig.MacToolPath}",
+	}
 	darwinX8664Rustflags = []string{}
 	darwinX8664Linkflags = []string{}
 )