Thanks for the bug report. Please try the attached patch; if it works around the 
bug for you, I'll install it on the emacs-26 branch and it should appear in the 
next Emacs release.

It would also be helpful to file a bug report with the Alpha Linux kernel folks, 
as there does seem to be a kernel bug here (or at least, an unexpected 
inconsistency with x86-64 with respect to how process personalities survive exec).