Generate prologue/epilogue using STRD/LDRD in Thumb mode, when tuning prefer_ldrd_strd flag is set, such as in Cortex-A15.
[1/4] New RTL patterns for LDRD/STRD in Thumb mode [2/4] Prologue using STRD in Thumb mode [3/4] Epilogue using LDRD in Thumb mode [4/4] Adjust tests gcc.target/arm/pr40457-*.c Testing and benchmarking: * No regression on qemu for arm-none-eabi cortex-a15 neon softfp thumb. * Successful bootstrap on Cortex-A15. * 3% performance improvement in a popular embedded benchmark. Ok for trunk? Thanks, Greta
