selftests: new x86 breakpoints selftest

Bring a first selftest in the relevant directory.  This tests several
combinations of breakpoints and watchpoints in x86, as well as icebp traps
and int3 traps.  Given the amount of breakpoint regressions we raised
after we merged the generic breakpoint infrastructure, such selftest
became necessary and can still serve today as a basis for new patches that
touch the do_debug() path.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Jason Wessel <jason.wessel@windriver.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Michal Marek <mmarek@suse.cz>
Cc: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>