Detecting hard float/version in pkgsrc

[coypu%SDF.ORG@localhost]

A good number of packages seem to default a worse case of hard float as
opposed to checking.

e.g.:
- lang/go/version.mk, assumes VFPv1 (GOARM=6 default). it could use VFPv3
for newer hardware, and it would probably not work for softfloat.

they will likely transition to GOARM=7 default at some point, and in the
absence of detection, it might be broken for older hardware.

- x11/pixman/Makefile is an example of all i386 being penalized for most
machines not having SSE2.

Others example of hacks exist, e.g. -mstackrealign for Firefox on i386,
but it is probably only needed for non-SSE2 machines. Or ffmpeg.

It might be nicer to have fancier float checks than just using
MACHINE_ARCH and assuming the worst case scenario when such hacks are
needed.

Opinions?