[PATCH] Fix PR64839: libsanitizer shouldn't require (commited)

[Yury Gribov]

Hi all,

I've submitted a libsanitizer cherry-pick from LLVM to not require xdr.h 
on systems where it's not available. Pre-approved by Jakub in 
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64839 .

-Y