minidump_dump: bug fixes.

 - Convert time_t values to UTC correctly. It is incorrect to cast a uint32_t*
   to time_t* because the two types may have different widths. This is the
   case on many 64-bit systems, where time_t is a 64-bit signed integer.
   Conversion is unified in a single function, and additional uses of time_t
   in minidump files not previously displayed in UTC are now displayed.
 - Interpret the IMAGE_DEBUG_MISC structure correctly.
 - When printing MINIDUMP_SYSTEM_INFO structures, always show the "x86" side
   of the union, and state whether it's expected to be valid. (Existing
   Breakpad-produced non-Windows minidumps for x86_64 use the "x86" side of
   union, but Windows minidumps for x86_64 use the "other" side, so I want to
   print both.)

R=ivanpe@chromium.org

Review URL: https://breakpad.appspot.com/5674002

git-svn-id: http://google-breakpad.googlecode.com/svn/trunk@1339 4c0a9323-5329-0410-9bdc-e9ce6186880e