llvm-objdump: Fix crash bug with printing unwind info on stripped file.

The current COFF unwind printer tries to print SEH handler function names,
assuming that it can always find function names in string table. It crashes
if file being read has no symbol table (i.e. executable).

With this patch, llvm-objdump prints SEH handler's RVA if there's no symbol
table entry for that RVA.

llvm-svn: 202466