libgomp testsuite: Only use 'blddir' if set

(It is unclear to me why the current working directory needs to be in
'LD_LIBRARY_PATH'; leaving that alone for now.)

	libgomp/
	* testsuite/lib/libgomp.exp (libgomp_init): Only use 'blddir' if
	set.
	* testsuite/libgomp.c++/c++.exp: Likewise.
	* testsuite/libgomp.oacc-c++/c++.exp: Likewise.