armstubs: Always use gcc compiler

it uses assembly files .S which needs to be processed with gcc alone
until they are made portable use gcc

Fixes
| armstub7.S:175:2: error: instruction requires: thumb
|  cbz r6, 9f @ Skip this section if we are core 0
|  ^

Signed-off-by: Khem Raj <raj.khem@gmail.com>

recipes-bsp/armstubs/armstubs.bb

@@ -34,3 +34,5 @@ addtask deploy before do_build after do_install
 do_deploy[dirs] += "${DEPLOYDIR}/${PN}"
 
 PACKAGE_ARCH = "${MACHINE_ARCH}"
+
+TOOLCHAIN = "gcc"