BaseTools/tools_def ARM: Make choice for soft float ABI explicit

BaseTools/tools_def ARM: Make choice for soft float ABI explicit

Recent GCC for ARM will complain when selecting the hard float ABI
without specifying the FPU implementation, even when just running the
preprocessor.

This all happens under the hood, and we never bothered in the past,
given that we don't emit floating point code anyway. However, to placate
newer compilers, make it explicit that the floating point ABI is always the
softfloat one, by moving the -msoft-float compiler option to
PLATFORM_FLAGS.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>