RFR: JDK-8077824: Introduce DefineNativeToolchain to handle toolchain configurations

[Erik Joelsson]

The macro SetupNativeCompilation handles various overrides of the 
compiler and linker used for compilation. There is also a LANG option, 
which implies that it needs to be told if it's compiling C or C++ when 
in reality, this is handled automatically. The real purpose of the LANG 
option is just to decide which executable to use for linking in certain 
conditions/platforms.

I would like to put some more structure around this by introducing a new 
macro, DefineNativeToolchain. There s a default and a couple of others, 
extending the default, for linking with the C++ compiler, building for 
the build platform etc. These definitions will help ensure that all the 
necessary executables and options are overridden for each of these usecases.

Bug: https://bugs.openjdk.java.net/browse/JDK-8077824

Webrevs:

The implementation is in NativeCompilation.gmk. I took the liberty of 
cleaning out some unused compiler/linker variables.
Top: http://cr.openjdk.java.net/~erikj/8077824/webrev.root.01/

The jdk changes are mostly just removing "LANG:=" and replacing it when 
necessary with "TOOLCHAIN:=TOOLCHAIN_LINK_CXX". Note that this is not 
needed for Windows only configurations.
Jdk: http://cr.openjdk.java.net/~erikj/8077824/webrev.jdk.01/

/Erik