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/global.go

@@ -54,6 +54,9 @@ var (
 		"-Wl,--pack-dyn-relocs=android+relr",
 		"-Wl,--no-undefined",
 		"-Wl,--hash-style=gnu",
+
+		"-B${ccConfig.ClangBin}",
+		"-fuse-ld=lld",
 	}
 )
 
@@ -80,7 +83,7 @@ func init() {
 
 	pctx.ImportAs("ccConfig", "android/soong/cc/config")
 	pctx.StaticVariable("RustLinker", "${ccConfig.ClangBin}/clang++")
-	pctx.StaticVariable("RustLinkerArgs", "-B ${ccConfig.ClangBin} -fuse-ld=lld")
+	pctx.StaticVariable("RustLinkerArgs", "")
 
 	pctx.StaticVariable("DeviceGlobalLinkFlags", strings.Join(deviceGlobalLinkFlags, " "))
 

rust/config/x86_darwin_host.go

@@ -22,7 +22,9 @@ import (
 
 var (
 	DarwinRustFlags     = []string{}
-	DarwinRustLinkFlags  = []string{}
+	DarwinRustLinkFlags = []string{
+		"-B${ccConfig.MacToolPath}",
+	}
 	darwinX8664Rustflags = []string{}
 	darwinX8664Linkflags = []string{}
 )

rust/config/x86_linux_host.go

@@ -22,7 +22,10 @@ import (
 
 var (
 	LinuxRustFlags     = []string{}
-	LinuxRustLinkFlags  = []string{}
+	LinuxRustLinkFlags = []string{
+		"-B${ccConfig.ClangBin}",
+		"-fuse-ld=lld",
+	}
 	linuxX86Rustflags   = []string{}
 	linuxX86Linkflags   = []string{}
 	linuxX8664Rustflags = []string{}