cmake: Use GCC CMSIS assembly with ARM toolchain

CMSIS provides both GNU and legacy ARM assembly files. Instead of using
armasm and legacy asm with ARM Compiler 6, CMakeLists is modified to
use the GNU format with both GCC_ARM and ARM C6.

Author: wernerlewis

cmsis/CMSIS_5/CMSIS/RTOS2/RTX/CMakeLists.txt

@@ -39,11 +39,7 @@ if(${CMAKE_CROSSCOMPILING})
 
     _mbed_get_cortex_a_exception_handlers()
 
-    if(${MBED_TOOLCHAIN} STREQUAL "GCC_ARM")
-        _mbed_get_cortex_m_exception_handlers(TOOLCHAIN_GCC)
-    elseif(${MBED_TOOLCHAIN} STREQUAL "ARM")
-        _mbed_get_cortex_m_exception_handlers(TOOLCHAIN_ARM)
-    endif()
+    _mbed_get_cortex_m_exception_handlers(TOOLCHAIN_GCC)
 endif()
 
 target_include_directories(mbed-rtos