crossgcc: Clean out ABI variable for GMP

This is sometimes set by packaging systems (eg Gentoo), so give it a
sane preset.

Change-Id: I651fad12128143e8ed5053e7e9871ea271bfc797
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: https://review.coreboot.org/20632
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

util/crossgcc/buildgcc

@@ -1022,6 +1022,9 @@ fi
 # Set up host compiler and flags needed for various OSes
 
 if is_package_enabled "GCC"; then
+# sane preset: let the configure script figure out things by itself
+# more importantly, avoid any values that might already linger in the variable
+OPTIONS="ABI="
 if [ $UNAME = "Darwin" ]; then
 	#GCC_OPTIONS="$GCC_OPTIONS --enable-threads=posix"