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_linux_host.go

@@ -21,8 +21,11 @@ import (
 )
 
 var (
-	LinuxRustFlags      = []string{}
-	LinuxRustLinkFlags  = []string{}
+	LinuxRustFlags     = []string{}
+	LinuxRustLinkFlags = []string{
+		"-B${ccConfig.ClangBin}",
+		"-fuse-ld=lld",
+	}
 	linuxX86Rustflags   = []string{}
 	linuxX86Linkflags   = []string{}
 	linuxX8664Rustflags = []string{}