8267246: -XX:MaxRAMPercentage=0 is unreasonable for jtreg tests on many-core machines

Reviewed-by: shade, erikj

Author: DamonFool

make/RunTests.gmk

@@ -738,7 +738,7 @@ define SetupRunJtregTestBody
 
   # Make sure MaxRAMPercentage is high enough to not cause OOM or swapping since
   # we may end up with a lot of JVM's
-  $1_JTREG_MAX_RAM_PERCENTAGE := $$(shell $$(EXPR) 25 / $$($1_JTREG_JOBS))
+  $1_JTREG_MAX_RAM_PERCENTAGE := $$(shell $(AWK) 'BEGIN { print 25 / $$($1_JTREG_JOBS); }')
 
   JTREG_TIMEOUT_FACTOR ?= 4