[llvm-objdump] - Do not include reserved undefined symbol in -t output.

This is https://bugs.llvm.org/show_bug.cgi?id=26892,

GNU objdump hides the special symbol entry:

SYMBOL TABLE:
000000000000a7e0 l     F .text	00000000000003f9 bi_copymodules
while llvm-objdump does not:

SYMBOL TABLE:
0000000000000000         *UND*		 00000000 
000000000000a7e0 l     F .text		 000003f9 bi_copymodules

Patch makes the behavior of the llvm-objdump to be consistent with the GNU objdump.

Differential revision: https://reviews.llvm.org/D56076

llvm-svn: 350840