llmv-objdump/COFF: Print export table contents.

This patch adds the capability to dump export table contents. An example
output is this:

  Export Table:
   Ordinal      RVA  Name
         5   0x2008  exportfn1
         6   0x2010  exportfn2

By adding this feature to llvm-objdump, we will be able to use it to check
export table contents in LLD's tests. Currently we are doing binary
comparison in the tests, which is fragile and not readable to humans.