Build: Find GNUARM compiler via find_program()

This makes it work better on Windows, which needs .exe in the filename,
and also makes it easier to detect errors with the toolchain.

Change-Id: Ic6492b6cea23b81bf521daa9f2953544f012b607
Signed-off-by: Øyvind Rønningstad <oyvind.ronningstad@nordicsemi.no>
diff --git a/toolchain_GNUARM.cmake b/toolchain_GNUARM.cmake
index 92acbf9..e100ab6 100644
--- a/toolchain_GNUARM.cmake
+++ b/toolchain_GNUARM.cmake
@@ -13,8 +13,13 @@
 
 set(CMAKE_SYSTEM_NAME Generic)
 
-set(CMAKE_C_COMPILER ${CROSS_COMPILE}-gcc)
-set(CMAKE_ASM_COMPILER ${CROSS_COMPILE}-gcc)
+find_program(CMAKE_C_COMPILER ${CROSS_COMPILE}-gcc)
+
+if(CMAKE_C_COMPILER STREQUAL "CMAKE_C_COMPILER-NOTFOUND")
+    message(FATAL_ERROR "Could not find compiler: '${CROSS_COMPILE}-gcc'")
+endif()
+
+set(CMAKE_ASM_COMPILER ${CMAKE_C_COMPILER})
 
 set(LINKER_VENEER_OUTPUT_FLAG -Wl,--cmse-implib,--out-implib=)
 set(COMPILER_CMSE_FLAG -mcmse)