libnss: don't use target CFLAGS with host toolchain

The OPTIMIZER variable is used to construct CFLAGS for host toolchain.
This breaks the build since we set it to TARGET_CFLAGS, and these may not be
supported by host toolchain. Augment the cross-compile patch to handle
OPTIMIZER so that we can initialize it to TARGET_OPTIMIZER, and override it
when used with host toolchain.

Fixes:
http://autobuild.buildroot.net/results/3f1/3f1afc1b58cb6fe53c438b55f169e2a78238806d/



Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>