Fix a configuring error with "-Xmx2048M: command not found"

A shell command error was observed during the jdk configuration.
```
checking flags for boot jdk java command for small workloads... -XX:+UseSerialGC -Xms32M -Xmx512M -XX:TieredStopAtLevel=1
/Users/fool/workspace/open/jdk/build/.configure-support/generated-configure.sh: line 91505: -Xmx2048M: command not found
checking whether to use sjavac... disabled, default
```

It might be fixed by
```
diff -r 10f11578b1f4 make/autoconf/build-performance.m4
--- a/make/autoconf/build-performance.m4 Fri Mar 13 15:36:50 2020 -0400
+++ b/make/autoconf/build-performance.m4 Sat Mar 14 08:01:39 2020 +0800
@@ -434,7 +434,7 @@
   fi
 
   JAVAC_SERVER_AVAILABLE=true
- SJAVAC_MEMORY_OPT=-Xms${MS_VALUE}M -Xmx${MX_VALUE}M
+ SJAVAC_MEMORY_OPT="-Xms${MS_VALUE}M -Xmx${MX_VALUE}M"
   UTIL_ADD_JVM_ARG_IF_OK([$SJAVAC_MEMORY_OPT],SJAVAC_SERVER_JAVA_FLAGS,[$SJAVAC_SERVER_JAVA])
   if test "x$JVM_ARG_OK" = "xfalse"; then
     AC_MSG_WARN([Could not set '$SJAVAC_MEMORY_OPT' on bootjdk, disabling sjavac and javac server])
```