Unbreak build with base libstdc++ on i386 and powerpc

std::abs(int64_t) for 32bit archs or std::llabs are not available in
base libstdc++ from forked GCC 4.2.1 or before 10.0-RELEASE (r255294)
for libstdc++ in lang/gcc47 and earlier (upstream PR libstdc++/54686).
To avoid the build error use mozilla::Abs which implements overloads
for all signed natural types but has unsigned return value.

This change won't be requried for Firefox 36 et al.

PR: 196772
Reported by: many
Submitted by: bar (initial version)
Obtained from: https://github.com/mozilla/gecko-dev/commit/e664fe8
Differential Revision: https://reviews.freebsd.org/D1569
Approved by: flo (mentor)
MFH: 2015Q1