Fix mismatched visibility of assembler impls of StgRun

Based on a patch from PHO, in trac #7813.

We were previously declaring StgRun as a private symbol (in
rts/StgRun.h), but were actually defining it as a public one (in
rts/StgCRun.c). This caused a linkage problem with old binutils. See:
http://www.haskell.org/pipermail/ghc-devs/2013-April/000932.html