For Solaris with old gcc versions (that don't understand

For Solaris with old gcc versions (that don't understand
-mcpu=ultrasparc), don't silently step to solaris-sparc-gcc, which
would unnecessarily use less efficient assembler code.
Instead, use new configuration variant solaris-usparc-oldgcc;
this also means that someone who reads the output of ./config will
notice that it may be advantegeous to upgrade gcc.