[AVR][MC] Emit some aliases for GPRs and IO registers

Emit the following aliases (if available):

.set __tmp_reg__, [0|16]
.set __zero_reg__, [1|17]
.set __SREG__, 63
.set __SP_H__, 62
.set __SP_L__, 61
.set __EIND__, 60
.set __RAMPZ__, 59

Reviewed By: aykevl

Differential Revision: https://reviews.llvm.org/D119807