glGet, 64b version

Implemented glGetInteger[64i_v,64v], which
is very similar to the 32b version, except
it has a 64b output argument.

Change-Id: I1874109da419b82b2499f4d82d9c46ab44381d65
Reviewed-on: https://swiftshader-review.googlesource.com/2864
Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>