[Bug go/83308] Missing platform definitions for SH in libgo

[ian at airs dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83308

--- Comment #20 from Ian Lance Taylor <ian at airs dot com> ---
This will be in GCC 8.

Backports to GCC 7 are fine with me but I'm not going to do them myself.