[Builtin] [ARM] Update CMake to support the build of armv6m

Summary:
Setting -DCOMPILER_RT_TEST_TARGET_TRIPLE=armv6m-none-eabi  will enable the build of builtin functions ARMv6m.
Currently, only those asms that support armv6m are added.

TODO:All asm sin ARM_EABI_Sources are ported for thumb1 so Thumb1_EABI_Sources will be deprecated.

Reviewers: rengolin, compnerd

Reviewed By: compnerd

Subscribers: aemerson, mgorny, llvm-commits

Differential Revision: https://reviews.llvm.org/D28463

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@292504 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/cmake/base-config-ix.cmake b/cmake/base-config-ix.cmake
index 71d1499..6f9f151 100644
--- a/cmake/base-config-ix.cmake
+++ b/cmake/base-config-ix.cmake
@@ -172,6 +172,7 @@
       else()
         test_target_arch(arm "" "-march=armv7-a" "-mfloat-abi=soft")
         test_target_arch(armhf "" "-march=armv7-a" "-mfloat-abi=hard")
+        test_target_arch(armv6m "" "-march=armv6m" "-mfloat-abi=soft")
       endif()
     elseif("${COMPILER_RT_DEFAULT_TARGET_ARCH}" MATCHES "aarch32")
       test_target_arch(aarch32 "" "-march=armv8-a")