All builds using ASAN_CFLAGS were with Asan but no optimisation, making
them particularly slow. Indeed, we were overwriting CFLAGS which
defaults to -O2 and not using any -O in the replacement. (CMake already has
optimisations on with ASan.)
While at it, also remove -Wall -Wextra which are redundant as they are
already part of WARNING_CFLAGS which we are not overwriting.
Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
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