Call target_can_do_single_step from maybe_software_singlestep

When the PikeOS osabi sniffer was added, Pedro suggested that a target
could omit stepping from its vCont? reply packet to tell gdb that
software single-step must be used:

https://sourceware.org/legacy-ml/gdb-patches/2018-09/msg00312.html

This patch implements this idea by moving the call to
target_can_do_single_step into maybe_software_singlestep.

I've also removed some FIXME comments from gdbarch_components.py, and
slightly updated the documentation for gdbarch_software_single_step.
I think these comments are somewhat obsolete now that
target_can_do_single_step exists -- the current approach isn't exactly
what the comments intended, but on the other hand, it exists and
works.

Following review comments from Andrew, this version changes
record-full to use maybe_software_singlestep, and then combines
maybe_software_singlestep with insert_single_step_breakpoint.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=28440

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