Refix _start in os_solaris.cpp so it doesn't give
compilation warning or error on amd64 (or anywhere else).

When we switched to SS10 the compilers noticed that there's another
extern "C" version of _start declared with a different overloading. It
gives a warning that you can't over load "C" linkage functions, but we need
our linkage for thr_start() call.

It seems that void _start() is declared in the header file
/usr/include/sys/segments.h for something else.

Also fix the os_linux.cpp start to be consistent so that someday the os files can be merged.