Changing the ARM variant reported by 'uname -m'

[Tuomas Tynkkynen]

Hi,

For our distro (NixOS), I've experimented with building packages for 
ARMv6 on ARMv7 hardware. This has generally worked fine, except that 
some build scripts (in coreutils, for example) notice that `uname -m` 
returns 'armv7l' and thus decide to e.g. add some ARMv7-specific gcc 
flags, which makes the binaries not run on actual ARMv6 devices.

So to avoid that, it would be necessary to have a way to make `uname -m` 
return a different ARM architecture version that the hardware has. After 
all, there is precedent for doing this: calling personality(PER_LINUX32) 
turns 'x86_64' to 'i686' and so on for other architectures.

I can see two ways of implementing this:

1) Keep using the personality() call by reserving some PER_* constants 
for architectures to use and decide what to map them to. E.g. the ARM 
implementation would require 8 values for choosing one of:
       armv4 armv4t armv5t armv5te armv5tej armv6 armv7 armv7m
plus automatically appending 'b' or 'l' depending on endianness.

2) Add a new setmachine() system call that allows setting the machine 
   name to an arbitrary string inside a non-initial UTS namespace.