[dev.power64] gc: fix etype of strings

[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://golang.org/cl/170100043