Fix getRelocationValueString to return the symbol name for EM_386.

Summary: This helps llvm-objdump -r to print out the symbol name along
with the relocation type on x86. Adjust existing tests from checking
for "Unknown" to check for the symbol now.

Test Plan: Adjusted test/Object tests.

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D5987

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220866 91177308-0d34-0410-b5e6-96231b3b80d8