glibc: Consider adding -Wno-error in cases when not using -O2

glibc: Consider adding -Wno-error in cases when not using -O2

glibc has recently turned on Werror globally which is good but then not
all option combos are well tested so there still remains cleanup needed
when not using -O2, so lets just disable Werror in such cases, until
fixed upstream

Change-Id: I2d491c360a15b0752c97ff77ee0faaeede6e8d2a
Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>