You can use CrossStudio for ARM with a third party supplied ARM GCC toolchain. To do this you must set the project properties Build > Use External GCC to Yes, the Build > GCC Prefix to arm-none-eabi- and Build > Tool Chain Directory to the directory containing the gcc executable for example C:/Program Files (x86)/GNU Tools ARM Embedded/4.7 2012q4/bin.

To be able to use the code completion and source navigation features you can set the project property Source Code > Additional Code Completion Compiler Options to specify the directories that the ARM GCC toolchain will access for example -isystemC:/Program Files (x86)/GNU Tools ARM Embedded/4.7 2012q4/arm-none-eabi/include.