[Driver] Enable __cxa_atexit on Solaris

Starting with Solaris 11.4 (which is now the required minimal version), Solaris does
support __cxa_atexit.  This patch reflects that.

One might consider removing the affected tests altogether instead of inverting them,
as is done on other targets.

Besides, this lets two ASan tests PASS:

  AddressSanitizer-i386-sunos :: TestCases/init-order-atexit.cc
  AddressSanitizer-i386-sunos-dynamic :: TestCases/init-order-atexit.cc

Tested on x86_64-pc-solaris2.11 and sparcv9-sun-solaris2.11.

Differential Revision: https://reviews.llvm.org/D64491

llvm-svn: 366305