cmake: assembler optimization options

Commit f499ea0a made it so that the compiler optimization flags are also
passed to the assembler. However, this breaks for the IAR toolchain
since it uses the GNU assembler which does not have the same
optimization flags as the IAR compiler.

This commit separates the compiler and assembler optimizations to make
it possible to specify appropriate flags in a toolchains corresponding
`compiler_flags.cmake`

Signed-off-by: Daniel Flodin <daniel.flodin@iar.com>