Add LOCAL_NOSANITIZE.

This can be used to selectively disable individual sanitizers on a
target. For example, some parts of libc should be built with
SafeStack (when requested with SANITIZE_TARGET), but never with
AddressSanitizer. Current build rules specify LOCAL_SANITIZE := never
to disable AddressSanitizer; the idea is to change that to
LOCAL_NOSANITIZE := address thread.

Bug: 27729263
Change-Id: I2b770f2ce3faf6ad6798792327e96adb86fe4a4f