runtime: Copy runtime_panicstring from master library.

libgo/runtime/go-append.c

libgo/runtime/go-close.c

libgo/runtime/go-eface-compare.c

libgo/runtime/go-eface-val-compare.c

libgo/runtime/go-interface-eface-compare.c

libgo/runtime/go-make-slice.c

libgo/runtime/go-map-delete.c

libgo/runtime/go-map-index.c

libgo/runtime/go-new-channel.c

libgo/runtime/go-new-map.c

libgo/runtime/go-panic.c

libgo/runtime/go-panic.h

libgo/runtime/go-reflect-map.c

libgo/runtime/go-reflect.c

libgo/runtime/go-runtime-error.c

libgo/runtime/go-send-nb-small.c

libgo/runtime/go-send-small.c

libgo/runtime/go-signal.c

libgo/runtime/go-strslice.c

libgo/runtime/go-type-eface.c