eco32: add support for 'H' operand modifier

This allows for inline assembler constructs like this
asm("stw %H0,$0,0" : : "r"(reg));
which will use the high part of a double word register pair.
eco32: remove sign_extend from mulsi3

Since the match_operands are already SI, there is no need to
sign_extend them to SI.
Besides, removing this fixes an ICE in simplify_unary_operation_1.