[dev.cc] cmd/8g: work around "out of fixed registers" on Plan 9

This change works around the "out of fixed registers"
issue with the Plan 9 C compiler on 386, introduced by
the Bits change to uint64 in CL 169060043.

The purpose of this CL is to be able to properly
follow the conversion of the Plan 9 runtime to Go
on the Plan 9 builders.

This CL could be reverted once the Go compilers will
be converted to Go.

Thanks to Nick Owens for investigating this issue.

LGTM=rsc
R=rsc
CC=austin, golang-codereviews, mischief
https://codereview.appspot.com/177860043