filesystem_config: Allow executables in /system/libexec

Files in $PREFIX/libexec, such as gcc's cc1, need to be executable.
Don't let make_ext4fs strip executable permissions from them.

Change-Id: Ieb90b237ba54e38deaa1705777414d207d0bae8e
Signed-off-by: Bernhard Rosenkraenzer <Bernhard.Rosenkranzer@linaro.org>