lib.mk: make sure archive file contains only specified objects
lib.mk: make sure archive file contains only specified objects

The recipe for archive (.a) files is updated so we won't try to append
objects to an existing archive left over from a previous build.

This fixes the following build error:
 $ make -s PLATFORM=hikey
 $ make -s PLATFORM=hikey CFG_ARM64_core=y \
     CROSS_COMPILE_core=aarch64-linux-gnu-
 [...]libtomcrypt.a: error adding symbols: File in wrong format

Signed-off-by: Jerome Forissier <ced78d268b4c9a0a20a59a9feeebd2ad90da03d3@linaro.org>
Reviewed-by: Pascal Brand <0b55292d2562685bad4ce52d64f802e8f1cd299e@linaro.org>

cedric-chaumont-st-dev/optee_os,cedric-chaumont-st-dev/optee_os,matt2048/optee_os,Microsoft/optee_os,Microsoft/optee_os,pascal-brand-st-dev/optee_os,pascal-brand-st-dev/optee_os,Microsoft/optee_os,cedric-chaumont-st-dev/optee_os,pascal-brand-st-dev/optee_os,pascal-brand-st-dev/optee_os,Microsoft/optee_os,pascal-brand-st-dev/optee_os,matt2048/optee_os,matt2048/optee_os,Microsoft/optee_os,matt2048/optee_os,cedric-chaumont-st-dev/optee_os