BPF: explicitly specify bpfel triple for certain tests

Commit 54d9f743 ("BPF: move AbstractMemberAccess and
PreserveDIType passes to EP_EarlyAsPossible") changed most
of CORE tests with opt run followed by llc and opt requires
the target triple specified in the IR.

There are few tests where little endian and big endian will
report different result and for little endian versions of
tests, "target triple = "bpf"" will produce wrong results
if the test executed in a big endian machine, e.g.
PowerPC big endian machine, since target "bpf" represents
host endian and will resolve to "bpfeb".
The builtbot reported such failures when build-and-run
on a PowerPC big endian machine.

To fix the issue, using "target triple = "bpfel"" instead.