x86: correct build dependencies in opcodes/

With the command in the rule merely being "echo", i386-tbl.h won't be
rebuilt if missing, when at the same time i386-init.h is present and
up-to-date. Use a pattern rule instead to express the multiple targets
correctly (the &: rule separator is supported only by GNU make 4.3 and
newer). Note that now, for the opposite case to work (i386-tbl.h is
up-to-date but i386-init.h is missing), i386-init.h also needs
mentioning as a dependency somewhere: Add a fake dependency for
i386-opc.lo ("fake" because i386-opc.c doesn't include that header).

At the same time use $(AM_V_GEN) in the actual rule, replacing the
earlier (open-coded) "echo". And while there also drop a duplicate
dependency of i386-gen.o on i386-opc.h.