Halve memory used by scanPerf performance test (#27873)

[reviewed by @e-kayrakli and OK'd by @benharsh]

By default, this test uses a problem size that consumes ~1/2 of the
physical memory by using '4' as the default memFraction and allocating
two arrays of that size. This PR divides the problem size by 1/2 again
to support portability to platforms that only support registering 50% of
the heap memory.

Author: bradcray

test/scan/scanPerf.chpl

@@ -5,8 +5,8 @@ config const memFraction = 0;
 const totMem = here.physicalMemory(unit = MemUnits.Bytes);
 const defaultN = if memFraction == 0
                    then 30
-                   else numLocales * ((totMem / numBytes(int)) / memFraction /
-                   here.numColocales);
+                   else numLocales * ((totMem / numBytes(int)) /
+                                     (2*memFraction) / here.numColocales);
 
 config const n = defaultN,
              printTiming = false,