Consolidate arm 'standalone' clang flags into bsd.sys.mk

[dim]

Summary

In multiple places, clang flags meant for 'standalone' mode on arm
(disabling movt and fpu instructions) have been duplicated. Since clang
3.8.0 spells the flag to disable movt differently, this became yet a
little uglier.

So let's move the ugliness to bsd.sys.mk, where it (maybe) belongs.
Warner, I think you will have some ideas and directions for this?

In any case, this is simply a draft to get the process going.

[emaste]

I think this is fine, but would like ARM experts to approve.

[dim]

This patch needs a little rebasing, I'll do that later on.

[andrew]

lib/libstand/Makefile

49–51

Could you add this comment to bsd.sys.mk so we remember why the flags are needed.