tcg/i386: Add shortcuts for registers used in L constraint

tcg/i386: Add shortcuts for registers used in L constraint

While 64 bit hosts use the first three registers which are also used
as function input parameters, 32 bit hosts use TCG_REG_EAX and
TCG_REG_EDX which are not used in parameter passing.

After defining new register macros for the registers used in L
constraint, the patch replaces most occurrences of
tcg_target_call_iarg_regs[0], tcg_target_call_iarg_regs[1] and
tcg_target_call_iarg_regs[2] by those new macros.

tcg_target_call_iarg_regs remains unchanged when it is used for input
arguments (only with 64 bit hosts) before tcg_out_calli.

A comment related to those registers was fixed, too.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
[aurel32: build fix on i386, small optimization for i386 in the prologue]
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>