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.