Fix leak scan SEGV catcher when ptr starts in unreadable page (readable for aspacemgr)

The fault catcher installed during leak scan to catter e.g. for
possible desynchronisation between real protection and aspacemgr
was not activated when the scanned ptr was directly pointing in
a desynchronised page.
This was (initially only) visible on ppc32 (gcc110) as the page size of
gcc110 is big (64 K).

=> modified the leak-segv-jmp test so as to produce the problem also 
on systems with smaller pages.

The fix consists in calling the setjmp before the scan loop,
and skip the bad address which has been recorded by the fault
catcher.
Also, deemed better to just skip one single Addr rather than a full page
(e.g. to skip less data in case some addresses are unreadable e.g.
on strange hardware).

Performance of the leak scan has been measured, seems slightly
faster on x86,amd64 and ppc32. Slightly slower on ppc64.

Also if verbose argument is given, outputs the nr of bytes skipped
due to fault.



git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13623 a5019735-40e9-0310-863c-91ae7b9d1cf9