RFR: 8241034: Fix a configuring error with "-Xmx2048M: command not found"

[jiefu(傅杰)]

Hi all,

JBS:    https://bugs.openjdk.java.net/browse/JDK-8241034
Webrev: http://cr.openjdk.java.net/~jiefu/8241034/webrev.00/

A shell command error was observed during the jdk configuration.
  ```
  jdk/build/.configure-support/generated-configure.sh: line 91505: -Xmx2048M: command not found
  ```
To find out the error message, you can just grep "line 91505: -Xmx2048M: command not found" in the configure.log file.

It might be better to fix it.

Testing:
 - Configuring and building these configurations on macos
   * macosx-x86_64-server-fastdebug
   * macosx-x86_64-server-release
   * macosx-x86_64-minimal-release
   * macosx-x86_64-zero-release

Please review it and give me some advice.

Thanks a lot.
Best regards,
Jie