Global: Allow overriding float-abi flags

We currently default to "-mfloat-abi=hard -mfpu=fpv4-sp-d16" for M4F cores, and
and variations of "-mfloat-abi=soft" for the others. Keep the M4F default, and
move others to no FP flags for consistency, but allow overriding these flags
via the FP_FLAGS environment variable.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>