PMS (see URL), section 4.2.9 "package.mask" states that the -spec syntax can be used to invert a mask in a parent profile. default-linux/{amd64,x86}/package.mask tries to use this to invert a mask in the global profiles/package.mask which isn't a parent profile. If you really want to invert this in some profiles, the mask should be moved to profiles/base/package.mask where it then belongs. (PMS includes this restriction because Portage's behaviour when combining -lines with multiple parents in profiles is sufficiently bizarre that it's not reasonable to specify.
QA is free to move the mask, if they are sure it remains working the same and that the mask in the other place is put in first, so that it won't be unmasked on non-amd64/x86 for no time.
CC'ing qa per Halcy0n's request.
Moved the mask from the global package.mask to base/package.mask, embedded/package.mask and selinux/package.mask. It was all done in one commit, so it shouldn't be possible for them to have been unmasked on other archs at all (just so you know :) ) As always, if I did break something, please hit me with the cluebat and let me know.
