Earlyclobber inline asm input registers

[Prabhuk]

asm_arm.inc inline assembly functions such as uECC_vli_add has output registers written to before the input registers are fully consumed. The inline assembly constraints for those registers are underspecified and must include earlyclobber constraint "&".

Compiling with GCC for cortex-M4 does not create any problems even though the constraints are currently underspecified but could potentially break the code in the future.