ld/testsuite: skip bootstrap.exp when OFILES are missing

ld/testsuite: skip bootstrap.exp when OFILES are missing

OFILES are normally provided through an environment variable set by
Makefiles. However, when launching the testsuite directly through
runtest outside the build tree, it can be hard to retrieve them.
Thus, they can be missing.
Instead of letting tcl raise an error when trying to access this
OFILES variable, skip bootstrap.exp if it doesn't exist.

ld/ChangeLog:

	* testsuite/ld-bootstrap/bootstrap.exp: Skip if OFILES is
	missing