The probe we do to determine what flags to use to make the usermode
executables use a non-default text address has some flaws:
 * we run it even if we're not building the user binaries
 * we don't expect "ld --verbose" to fail

The combination of these two results in a harmless but
ugly "ld: unknown option: --verbose" message when running
configure on OSX.

Improve the probe to only run when we need it and to fail
nicely when even the backstop 'ld --verbose' approach fails.
Reported-by: NStefan Weil <sw@weilnetz.de>
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Reviewed-by: NStefan Weil <sw@weilnetz.de>
Message-id: 1466287502-18730-2-git-send-email-pmaydell@chiark.greenend.org.uk