cc2538: Enable workaround for Cortex-M3 erratum #602117

The core of the CC2538 SoC is the ARM Cortex-M3 r2p0, which is affected by the
erratum #602117 'LDRD with base in list may result in incorrect base register
when interrupted or faulted'. The conditions triggering this issue are not very
likely to occur, but as a matter of precaution, it is better to enable
GCC's -mfix-cortex-m3-ldrd option as a workaround, all the more it can only make
the compiler choose other CPU registers to use with LDRD.