[dev.power64] gc: fix etype of strings

The etype of references to strings was being incorrectly set
to TINT32 on all platforms.  Change it to TSTRING.  It seems
this doesn't matter for compilation, since x86 uses LEA
instructions to load string addresses and arm and power64
disassemble the string into its constituent pieces (with the
correct types), but it helps when debugging.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://codereview.appspot.com/170100043
[dev.power64] 9g: fix addr width calculation; enable MOV* width check

9g's naddr was missing assignments to a->width in several
cases, so the optimizer was getting bogus width information.
Add them.

This correct width information also lets us enable the width
check in gins for MOV*.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://codereview.appspot.com/167310043