[test] Fix baremetal test to allow any -resource-dir

The baremetal test (r303873) has been added with expectance of very
specific -resource-dir. However, the test itself nor the BareMetal
driver does not enforce any specific -resource-dir, making this
constraint invalid. It already has been altered twice -- in r303910 for
Windows compatibility, and in r304085 for systems using lib64. To
account for even more systems, just use [[RESOURCE_DIR]] like a number
of other tests do. This is needed for Gentoo where RESOURCE_DIR starts
with ../ (uses relative path to a parent directory).

Differential Revision: https://reviews.llvm.org/D33877

llvm-svn: 304715