Add POPCNT and TZCNT opcodes

[donovanhide]

Would be great if 6g had POPCNTQ/L/W and TZCNTQ/L/W instructions added.

Real world usage:

bits-and-blooms/bitset#19

[gopherbot]

Comment 1 by rayneolivetti:

Duplicate of #4816?
With TZCNT, do you mean LZCNT?

[donovanhide]

Comment 2:

Oops, sorry about the duplicate!
I do mean TZCNT (trailing zero count), but LZCNT would be useful too. 
http://software.intel.com/sites/products/documentation/doclib/iss/2013/compiler/cpp-lin/GUID-E6E297BB-E575-4525-AE1C-DFD7D5B662F5.htm
Probably not the best place to discuss, but CPUID support in the runtime for ARM,x86 and
AMD64 would be great too... Seems like every assembly routine rewrites their own version
of it!

[robpike]

Comment 3:

Status changed to Duplicate.

Merged into issue #4816.