RFR: JDK-8001912, JDK-8001913 Improve detection of msvcr100.dll

[Magnus Ihse Bursie]

Bug: https://bugs.openjdk.java.net/browse/JDK-8001912
Bug: https://bugs.openjdk.java.net/browse/JDK-8001913

To build on Windows, we need a redistribution copy of the Visual Studio 
runtime library msvcr100.dll. There is unfortunately no well-defined 
place to find this. Our current code searches for this slowly and 
inefficiently, and does not look in all reasonable places. Also, it does 
not check if the found DLL matches the architecture of the target cpu 
we're building for.

These issues were discussed in two bugs, but it turned out that they 
needed to be solved both at a single time, since we need to check the 
architecture at each potential DLL we find, instead of once at the end. 
(Otherwise we can't continue looking for a better match.) This is 
similar to how we locate Visual Studio or the Boot JDK.

I moved the msvcr setup logic to a new function, 
TOOLCHAIN_SETUP_MSVCR_DLL. WebRev does not display this gracefully, 
unfortunately.

When testing with --with-msvcr-dll, it turned out that we do not handle 
relative paths like ./myspeciallibs/msvcr100.dll correctly. This fix 
also addresses this, by adding -a (rewrite as absolute path) to all 
cygpath -u calls.

WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8001912-improve-msvcr100-dll-detection/webrev.01

/Magnus