neon: Enable Flush-to-zero mode for float programs

Some of the instructions are not NEON (which always uses FTZ),
but are actually VFP, which requires enabling FTZ mode.
4 jobs for neon in 1 minute and 16 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #736916
debian sid

00:00:42

passed #736919
docker windows 1607
msys2

00:01:08

passed #736917
docker windows 1607
vs2017 amd64

00:01:15

passed #736918
docker windows 1607
vs2017 x86

00:01:14