[test-suite] Avoid segfault in CLAMR test on non-Linux systems

On FreeBSD, the CLAMR test immediately segfaults on startup, when it
parses /proc/pid/status in the `memstats_memused()` function. This is
because the parser is not very robust, and the FreeBSD /proc format is
different than Linux.

I expect this code to crash on anything else than Linux, but for our
test purposes the memory statistics printed are not important, since
nobody ever looks at the produced output.

Since there is already a `MEMORY` define controlling the printing of
memory statistics, only call the `memstats_memused()` function if it is
explicitly enabled (which it is not, by default).

Reviewed By: brad

Differential Revision: https://reviews.llvm.org/D146387

Author: DimitryAndric

MultiSource/Benchmarks/DOE-ProxyApps-C++/CLAMR/clamr_cpuonly.cpp

@@ -271,8 +271,8 @@ int main(int argc, char **argv) {
    mesh->parallel_output("CPU:  setup time               time was",cpu_time_main_setup, 0, "s");
    #endif 
 
-   long long mem_used = memstats_memused();
    #ifdef MEMORY
+   long long mem_used = memstats_memused();
    if (mem_used > 0) {
       mesh->parallel_output("Memory used      in startup ",mem_used, 0, "kB");
       mesh->parallel_output("Memory peak      in startup ",memstats_mempeak(), 0, "kB");
@@ -627,8 +627,8 @@ extern "C" void do_calc(void)
       //  Get overall program timing.
       double elapsed_time = cpu_timer_stop(tstart);
 
-      long long mem_used = memstats_memused();
       #ifdef MEMORY
+      long long mem_used = memstats_memused();
       if (mem_used > 0) {
          printf("Memory used      %lld kB\n",mem_used);
          printf("Memory peak      %lld kB\n",memstats_mempeak());