multimedia/libvpx: update to 1.4.0.488 (snapshot)

Sync system libvpx with bundled libvpx in Firefox 40 in order
to fix vulnerabilities in the former.

While here fix other issues:

• Add powerpc targets removed by upstream
• Add sparc64 support [1]
• Improve gcc42 workaround [2]
• Strip binaries as reported by stage-qa
• Drop unused -no-integrated-as hack for Clang

Changes: https://chromium.googlesource.com/webm/libvpx/+log/v1.4.0..e67d45d
ACC report: https://people.freebsd.org/~jbeich/compat_reports/libvpx/1.4.0_to_1.4.0.488/compat_report.html
PR: 202270
PR: 186394 [1] (based on)
PR: 189136 [2]
Exp-run by: antoine
Approved by: ashish (maintainer)
MFH: 2015Q3
Security: http://www.vuxml.org/freebsd/34e60332-2448-4ed6-93f0-12713749f250.html