util/xcompile: Print fatal error messages to stderr instead of stdout

This uses die() which was previously unused.
Before this change an unhelpful error message was printed when make tried
to parse English text as if it was part of the makefile:
.xcompile:1: *** missing separator (did you mean TAB instead of 8 spaces?).  Stop.

After this change the first error message at least mentions that iasl is
missing:
ERROR: no iasl found
make: -print-libgcc-file-name: Command not found
make: -print-libgcc-file-name: Command not found
make: -print-libgcc-file-name: Command not found
/bin/sh: 0: Illegal option -
Makefile.inc:36: *** Please use the coreboot toolchain (or prove that your toolchain works).  Stop.

Change-Id: I79d5de5993e3828460130192df376daa55f32aa0
Signed-off-by: Daniele Forsi <dforsi@gmail.com>
Reviewed-on: http://review.coreboot.org/6272
Tested-by: build bot (Jenkins)
Reviewed-by: Martin Roth <gaumless@gmail.com>