[llvm-readobj][XCOFF] Fix the error dumping for the first

item of StringTable.

Summary: For the string table in XCOFF, the first 4 bytes
contains the length of the string table, so we should
print the string entries from fifth bytes. This patch
also adds tests for llvm-readobj dumping the string
table.

Reviewed By: jhenderson

Differential Revision: https://reviews.llvm.org/D105522