If we have zero initialized data with external linkage, use .zerofill to

If we have zero initialized data with external linkage, use .zerofill to
emit it (instead of .space), saving a bit of space in the .o file.

For example:
int foo[100];
int bar[100] = {};

when compiled with C++ or -fno-common results in shrinkage from 1160 to 360
bytes of space.  The X86 backend can also do this on darwin.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@26185 91177308-0d34-0410-b5e6-96231b3b80d8