buildgcc: Get the clean and correct uname on Cygwin

Running `uname` on Cygwin gets "CYGWIN_NT-6.1-WOW" instead of "Cygwin".
We need to fix the $UNAME on Cygwin.

Change-Id: I540bfc52089951006fd0e20bb9893a3d891df9e1
Signed-off-by: Zheng Bao <zheng.bao@amd.com>
Signed-off-by: Zheng Bao <fishbaozi@gmail.com>
Reviewed-on: http://review.coreboot.org/11124
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
diff --git a/util/crossgcc/buildgcc b/util/crossgcc/buildgcc
index 590e1f8..cfe881b 100755
--- a/util/crossgcc/buildgcc
+++ b/util/crossgcc/buildgcc
@@ -94,7 +94,7 @@
 CYAN='\033[1;36m'
 NC='\033[0m' # No Color
 
-UNAME=$(uname)
+UNAME=$(uname | grep -iq cygwin && echo Cygwin || uname)
 
 normalize_dirs()
 {