arch: ARM: set -mthumb, -mcpu, -mfpu and -mfloat-abi in linker flags

Some toolchains are built with multilib enabled in order to provide
multiple versions of the same library, optimized for different ABI
or architecture. They require the the ABI/architecture/CPU selection
options to be passed at linked time. This is important for example
when linking with newlib.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>