add ARCH_GPRS target

Arch GPRS uses LPC11U37_501
Here you have a tab instead of a space. It would be nice if you replace all tabs with 4 spaces instead.
Please refer to official coding style guidelines in section 'Rules' on our SDK developers' Wiki: http://mbed.org/teams/SDK-Development/wiki/mbed-sdk-coding-style -> "Indentation - 4 spaces. Please do not use tabs. "