Thumb2 inline ASM: IAR doesn't do register variables

IAR doesn't parse register variable declarations with specified
registers. IAR doesn't even honor the register keyword.
Can use small negative but IAR doesn't like it.
Specify the positive value instead.
Add a small code size version of mont_reduce_full using umlal and umaal.
Make 'asm' usage in variables use keyword '__asm__'.
Explicitly don't inline some functions when compiling with IAR.

Author: SparkiDev