[ARM] Fix test using thumb instead arm arch prefix

Currently, the ARM backend incorrectly parses every `arm` prefixed arch to be non-thumb, but `armv6m` is THUMB and doesnt have ARM ops causing the test to fail: `error: Function 'foo' uses ARM instructions, but the target does not support ARM mode execution.`

As a quick triage, this change switches the test to use `thumb`.

Uncovered by #151404

Author: tomershafir

clang/test/CodeGen/pr45476.cpp

@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 -triple armv6m-eabi -emit-llvm %s -o - | FileCheck -check-prefix=LIBCALL %s
+// RUN: %clang_cc1 -triple thumbv6m-eabi -emit-llvm %s -o - | FileCheck -check-prefix=LIBCALL %s
 // RUN: %clang_cc1 -triple armv8-eabi -emit-llvm %s -o - | FileCheck -check-prefix=NATIVE %s
 // PR45476