Remove extraneous characters when printing the timestamp for an XCOFF object file.

Summary:

The llvm-readobj prints extra characters for the timestamp when --file-headers is used with an XCOFF file. This change updates the format string used to print the time. In addition, the timestamp is printed in the local timezone, and a thread-safe call is used to convert the time.

Summit the patch on behalf of Stephen Peckham.

Reviewers: James Henderson, Digger Lin
Differential Revision: https://reviews.llvm.org/D144281