Build: Fix GCC version check

* Remove multiple version getters
* Get GCC version earliest possible

Signed-off-by: Dávid Házi <david.hazi@arm.com>
Change-Id: Idc87cbb8ed2a6f1b6d26ceece44f8d57ccc5400d
diff --git a/toolchain_GNUARM.cmake b/toolchain_GNUARM.cmake
index e47668e..320ede1 100644
--- a/toolchain_GNUARM.cmake
+++ b/toolchain_GNUARM.cmake
@@ -105,8 +105,6 @@
 
     if (DEFINED TFM_SYSTEM_DSP)
         # +nodsp modifier is only supported from GCC version 8.
-        # CMAKE_C_COMPILER_VERSION is not guaranteed to be defined.
-        EXECUTE_PROCESS( COMMAND ${CMAKE_C_COMPILER} -dumpversion OUTPUT_VARIABLE GCC_VERSION )
         if(GCC_VERSION VERSION_GREATER_EQUAL "8.0.0")
             # armv8.1-m.main arch does not have +nodsp option
             if ((NOT TFM_SYSTEM_ARCHITECTURE STREQUAL "armv8.1-m.main") AND
@@ -126,13 +124,13 @@
 endmacro()
 
 macro(tfm_toolchain_reload_compiler)
+    # CMAKE_C_COMPILER_VERSION is not guaranteed to be defined.
+    EXECUTE_PROCESS( COMMAND ${CMAKE_C_COMPILER} -dumpversion OUTPUT_VARIABLE GCC_VERSION )
+
     tfm_toolchain_set_processor_arch()
     tfm_toolchain_reset_compiler_flags()
     tfm_toolchain_reset_linker_flags()
 
-    # CMAKE_C_COMPILER_VERSION is not guaranteed to be defined.
-    EXECUTE_PROCESS( COMMAND ${CMAKE_C_COMPILER} -dumpversion OUTPUT_VARIABLE GCC_VERSION )
-
     if (GCC_VERSION VERSION_LESS 7.3.1)
         message(FATAL_ERROR "Please use newer GNU Arm compiler version starting from 7.3.1.")
     endif()