Switch to armv7 for Rust ARM

In the previous mode, building Rust code using certain intrinsics would
fail on arm32 since our runtime is armv7, but our codegen was arm.

Test: cd external/rust; mma
Bug: 169245712
Change-Id: Ie800486b39325486f5ff319e46632df2a6987391
diff --git a/rust/config/arm_device.go b/rust/config/arm_device.go
index ac4f1c6..adfe917 100644
--- a/rust/config/arm_device.go
+++ b/rust/config/arm_device.go
@@ -52,7 +52,7 @@
 }
 
 func (t *toolchainArm) RustTriple() string {
-	return "arm-linux-androideabi"
+	return "armv7-linux-androideabi"
 }
 
 func (t *toolchainArm) ToolchainLinkFlags() string {