valgrind: make ld-XXX.so strlen intercept optional

valgrind: make ld-XXX.so strlen intercept optional

Hack: Depending on how glibc was compiled (e.g. optimised
for size or built with _FORTIFY_SOURCE enabled) the strlen
symbol might not be found in ld-XXX.so. Therefore although
we should still try to intercept it, don't make it mandatory
to do so.

(From OE-Core rev: 84ec50e587e7464b260b1b189659b93b6dab0ef6)

Signed-off-by: Jackie Huang <jackie.huang@windriver.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>