It was added by:
http://gcc.gnu.org/ml/gcc-patches/2004-01/msg01111.html
But it was not documented. Zem no longer works at Apple now either.
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Summary: [4.0/4.1/4.2 Regression] attribute altivec is not
documented
Product: gcc
Version: 4.2.0
Status: UNCONFIRMED
Keywords: documentation
Severity: normal
Priority: P3
Component: target
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: pinskia at gcc dot gnu dot org
GCC target triplet: powerpc*-*-*, rs6000-*-*
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=27544