system.h: Add sparcv8plus to oracle/sunpro 32-bit detection

With specific compiler options selecting the arch like -xarch=sparc on
newer compilers like Oracle Studio 12.4 there is no definition of
__sparcv8 but __sparcv8plus which means the V9 ISA, but limited to the
32ÎíÎñbit subset defined by the V8plus ISA specification, without the
Visual Instruction Set (VIS), and without other implementation-specific
ISA extensions. So it should be the same as __sparcv8.

Closes https://github.com/curl/curl/pull/2491