cmake: remove CMAKE_ASM_FLAGS variable in toolchain file

As discussied in [YOCTO #14717] cmake contains a OEToolchainConfig.cmake
file to configure the toolchain correctly in cross-compile build for recipes
using cmake. The CMAKE_ASM_FLAGS are the configuration are meant for  assembly,
but the spelling is incorrect and the Flag is ASFLAGS for gcc and other compilers.
So this variable might neever have worked and it is better for
recipes to specify their own.

Signed-off-by: Martin Beeger <martin.beeger@online.de>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
(cherry picked from commit 72729ffb)
Signed-off-by: Steve Sakoman <steve@sakoman.com>