babeltrace2: Always use BFD linker when building tests with ld-is-lld distro feature

lld results in textrels in some .so used in tests, fixes

babeltrace2-ptest: ELF binary /usr/lib/babeltrace2/ptest/tests/lib/test-plugin-plugins/plugin-minimal.so has relocations in .text
babeltrace2-ptest: ELF binary /usr/lib/babeltrace2/ptest/tests/lib/test-plugin-plugins/plugin-sfs.so has relocations in .text [textrel]
ERROR: babeltrace2-2.0.5-r0 do_package_qa: QA Issue: babeltrace2: ELF binary /usr/lib/babeltrace2/plugins/babeltrace-plugin-ctf.so has relocations in .text
babeltrace2: ELF binary /usr/lib/babeltrace2/plugins/babeltrace-plugin-utils.so has relocations in .text
babeltrace2: ELF binary /usr/lib/babeltrace2/plugins/babeltrace-plugin-text.so has relocations in .text [textrel]

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>