RFR: JDK-8216021: RunTest.gmk might set concurrency level to 1 on Windows

[Erik Joelsson]

When running tests using "make run-test-prebuilt" on Windows, the 
calculation of available memory may fail. This only happens on certain 
systems, and is likely related to which version of Cygwin that is being 
used. The consequence is that concurrency gets set to 1. The cause of 
the problem is mishandling of Windows \r line endings, which Cygwin 
tools are known to handle differently in different versions.

To fix the problem here, I've added explicit filtering of \r before 
trying to interpret the memory size as a number.

I checked the corresponding construct in configure, and it seems to work 
on the same machine where RunTestsPrebuilt.gmk failed, so I left it alone.

Bug: https://bugs.openjdk.java.net/browse/JDK-8216021

Webrev: http://cr.openjdk.java.net/~erikj/8216021/webrev.01/

/Erik