Resolve GccTriple/ClangTriple early.

When these are Ninja variable they can't be used in android.Path since
they'll be rejected as invalid paths. We don't actually need them to be
a Ninja variable, so make them plain old strings instead.

Test: Still builds.
Change-Id: I04743cebd4b2f9be5685545f2ee941df180fce14
diff --git a/cc/x86_64_device.go b/cc/x86_64_device.go
index 6923f5f..2747cf0 100644
--- a/cc/x86_64_device.go
+++ b/cc/x86_64_device.go
@@ -140,8 +140,6 @@
 	pctx.SourcePathVariable("x86_64GccRoot",
 		"prebuilts/gcc/${HostPrebuiltTag}/x86/x86_64-linux-android-${x86_64GccVersion}")
 
-	pctx.StaticVariable("x86_64GccTriple", "x86_64-linux-android")
-
 	pctx.StaticVariable("x86_64ToolchainCflags", "-m64")
 	pctx.StaticVariable("x86_64ToolchainLdflags", "-m64")
 
@@ -179,7 +177,7 @@
 }
 
 func (t *toolchainX86_64) GccTriple() string {
-	return "${x86_64GccTriple}"
+	return "x86_64-linux-android"
 }
 
 func (t *toolchainX86_64) GccVersion() string {
@@ -211,7 +209,7 @@
 }
 
 func (t *toolchainX86_64) ClangTriple() string {
-	return "${x86_64GccTriple}"
+	return t.GccTriple()
 }
 
 func (t *toolchainX86_64) ToolchainClangLdflags() string {