Use md5sum as the build.ninja filename suffix if the suffix gets too long.

Use md5sum as the build.ninja filename suffix if the suffix is longer
than 64 characters. Also store the original suffix in a corresponding
.suf file for human lookup.
This fixes the "File name too long" error when kati tries to write to
build.ninja with the long original suffix.

Bug: 24302576
Change-Id: Ie04b0c14e1a9d593aa5c2a6b357b19a9e8a7158e