[llvm-ar] Use POSIX-specified timestamps for 'tv'.

Summary:
The POSIX spec says:

```
If the −t option is used with the −v option, the standard output format shall be:
"%s %u/%u %u %s %d %d:%d %d %s\n", <member mode>, <user ID>,
<group ID>, <number of bytes in member>,
<abbreviated month>, <day-of-month>, <hour>,
<minute>, <year>, <file>

where:

...
<abbreviated month>
Equivalent to the format of the %b conversion specification format in date.
<day-of-month>
Equivalent to the format of the %e conversion specification format in date.
<hour> Equivalent to the format of the %H conversion specification format in date.
<minute> Equivalent to the format of the %M conversion specification format in date.
<year> Equivalent to the format of the %Y conversion specification format in date.
```

This actually used to be the format printed by llvm-ar. It was apparently accidentally changed (see r207385 followed by comments in r207387). This makes it conform to GNU ar for easier replacement.

Reviewers: MaskRay

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D52940

llvm-svn: 343901