bpf: use PwdPrefix() helper

relPwd is only set if runtime.GOOS != "darwin" but is used
unconditionally. Instead, unconditionally set relPwd using the
cc.PwdPrefix() helper.

Bug: 238165437
Test: m timeInState.o
Signed-off-by: Connor O'Brien <connoro@google.com>
Change-Id: I5294bb6ea95dad60fbf64e9d958e1f1b32a02a48