[scudo] Temporary disabling the valloc test on armhf

Summary:
Weird failure where `errno != ENOMEM` on valloc failure. The returned pointer
is null since it passes the previous assert, so this shouldn't happen.

http://lab.llvm.org:8011/builders/clang-cmake-armv7-a15-full/builds/10931
http://lab.llvm.org:8011/builders/clang-cmake-thumbv7-a15-full-sh/builds/2469

Disabling until we figure out what's going on.

Reviewers: alekseyshl

Reviewed By: alekseyshl

Subscribers: aemerson, srhines, llvm-commits, kristof.beyls

Differential Revision: https://reviews.llvm.org/D38324

llvm-svn: 314344