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 #2491

include/curl/system.h

@@ -300,7 +300,9 @@
 
 #elif defined(__SUNPRO_C) /* Oracle Solaris Studio */
 #  if !defined(__LP64) && (defined(__ILP32) ||                          \
-                           defined(__i386) || defined(__sparcv8))
+                           defined(__i386) ||                           \
+                           defined(__sparcv8) ||                        \
+                           defined(__sparcv8plus))
 #    define CURL_TYPEOF_CURL_OFF_T     long long
 #    define CURL_FORMAT_CURL_OFF_T     "lld"
 #    define CURL_FORMAT_CURL_OFF_TU    "llu"