arm: Remove dead code

[imp]

Summary

This hasn't been used since we upgraded past gcc 4.6. Remove support for
compiling with compilers older than gcc 4.6 for arm atomics. And
generally for SYNC_ATOMICs that we had to use then (only arm still has
this code around).

[andrew]

It looks like this is used by compiler-rt, see R10:e1c0c6422a34f4159b9a9b1cef9c75f3aa8541a4

[imp]

Ah, that's different. I'll just punt