cmake: gcc: target_arm: Support advanced floating-point options

This commit updates the Zephyr build system to support specifying
advanced floating-point compilation options derived from the newly
introduced unified floating-point configurations.

The following changes are introduced by this commit:

1. Specify architecture floating-point option to the `-mcpu` flag.
2. Specify floating-point unit (FPU) type using the `-mfpu` flag.

Note that the `-march` flag is not specified separately because the
`-mcpu` flag provides more detailed architecture options and this
makes the `-march` flag redundant.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>