[ubsan][test] Don't disable ubsan testing on 64-bit Solaris/x86

Unlike asan, which isn't supported yet on 64-bit Solaris/x86, there's no reason to disable
ubsan.  This patch does that, but keeps the 64-bit ubsan-with-asan tests disabled.

Tested on x86_64-pc-solaris2.11.

Differential Revision: https://reviews.llvm.org/D63982

llvm-svn: 365302