Build: fix ARMClang _compiler_reload in cmake 3.18

Unset some more internal variables added in cmake 3.18 to allow ARMClang
compiler reloading to work correctly.

Change-Id: Ieff1e8b17057fc74bf426035e7baa1228a2a2394
Signed-off-by: Raef Coles <raef.coles@arm.com>
diff --git a/toolchain_ARMCLANG.cmake b/toolchain_ARMCLANG.cmake
index 94fe9db..f6ddaa5 100644
--- a/toolchain_ARMCLANG.cmake
+++ b/toolchain_ARMCLANG.cmake
@@ -128,6 +128,8 @@
     unset(CMAKE_C_LINK_FLAGS)
     unset(CMAKE_ASM_FLAGS_INIT)
     unset(CMAKE_ASM_LINK_FLAGS)
+    unset(__mcpu_flag_set)
+    unset(__march_flag_set)
 
     # cmake does not allow the use of +dsp / +nofpu syntax. An override is added
     # here to enable it. First reset the supported CPU list in case this has