ARM: 6778/1: compressed/head.S: make LDFLAGS_vmlinux into a recursively expanded variable

The simply expanded variable may be evaluated before the target file for
the stat command is up to date or even exists.  Switching to a recursively
expanded variable move the execution of the stat command to the location
where LDFLAGS_vmlinux is actually used, fixing the dependency issue
introduced by patch #6746/1.

Signed-off-by: Nicolas Pitre <nicolas.pitre@linaro.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>