PR21888 WIP: basic bpf variants of logging functions

PR21888 WIP: basic bpf variants of logging functions

The prior patch for PR23407 allows some progress on this.

Not everything works yet: PR23435 means some output could get
swallowed in a probe that calls exit(). Moreover assert() does
not work yet -- need to check what must be changed to allow
the string to be passed into a nested call.

* tapset/logging.stp (log): add bpf variant.
(warn): add basic bpf variant.
(error): actually print the error in bpf variant.

* testsuite/systemtap.bpf/bpf_tests/string2.stp: add testcase.