Adding GCC cross-compiler information to BaseTools folder

git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@4206 6f19259b-4bc3-4df7-8a09-765794883524

BaseTools/gcc/README.txt

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+You may run these scripts to build a Tiano Cross compiler. They have been
+tested on Cygwin, OS X and Linux. You should expect to hack on these scripts to
+make them work on your system. You may need to install additional tools on your
+system to make the scripts work.
+
+You will need
+
+  A recent version (3.0 or later should be fine) of gcc that is able to produce
+    executables for the machine that you want to run this compiler on (the host
+    machine).
+  wget or curl
+  tar
+  bzip
+  gzip
+  bash
+  and possibly others
+
+CYGWIN Notes
+
+You should setup cygwin to use binmode on all mounts. When you initially
+install cygwin it gives you the choice of Unix file mode (recommended) or DOS
+file mode. Unix mode will cause all the cygwin directories to be mounted in
+binmode, while DOS will mount the dirs in textmode. Here is an example of a
+cygwin install where the dirs are (properly) mounted in binmode.
+
+C:\cygwin\bin on /usr/bin type user (binmode)
+C:\cygwin\lib on /usr/lib type user (binmode)
+c:\workspace on /workspace type system (binmode)
+C:\cygwin on / type user (binmode)
+
+If you use textmode, it is likely that the build will fail in a way that is
+hard to debug.
+
+Cygwin is pretty slow, so it is not recommended for large builds.
+