[mips] Remove obstack.cc XFAIL added in r278504 since it broke X86 and PPC somehow.

Adding the XFAIL has caused msan to report a different line number in the call
stack (@LINE-3 rather than @LINE-30). The new line number looks more correct
at first glance since it's the line that uses uninitialized memory rather than
the first non-whitespace line of the file but this needs investigating.

llvm-svn: 278516