MFC 332906,332907,332976,333679,336053: Expand testing of breakpoints.

332906:
Extend support for ptrace() tests using breakpoints.

- Use a single list of platforms to define HAVE_BREAKPOINT for platforms
that expose a functional breakpoint() inline to userland. Replace
existing lists of platform tests with HAVE_BREAKPOINT instead.
- Add support for advancing PC past a breakpoint inserted via breakpoint()
to support the existing ptrace__PT_CONTINUE_different_thread test on
non-x86 platforms (x86 advances the PC past the breakpoint instruction,
but other platforms do not). This is implemented by defining a new
SKIP_BREAK macro which accepts a pointer to a 'struct reg' as its sole
argument and modifies the contents to advance the PC. The intention is
to use it in between PT_GETREGS and PT_SETREGS.

332907:
Expose breakpoint() to userland from <machine/cpufunc.h> on MIPS.

Enable ptrace() tests using breakpoint on MIPS as well.

332976:
Shorten some recently-added lines that are an extra indent over 80 columns.

333679:
Export a breakpoint() function to userland for riscv.

As a result, enable tests using breakpoint() on riscv.

336053:
Export a breakpoint() function to userland for arm and arm64.

Enable ptrace() tests using breakpoint() on these architectures.

Copy head@r302406 to stable/11 as part of the 11.0-RELEASE cycle.
Prune svn:mergeinfo from the new branch, as nothing has been merged
here.

Additional commits post-branch will follow.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Import RISC-V machine headers. This is a minimal set required to compile
kernel and userland.

Reviewed by: andrew, imp, kib
Sponsored by: DARPA, AFRL
Sponsored by: HEIF5
Differential Revision: https://reviews.freebsd.org/D4554