fix gdbsrv inferior calls when PT_GNU_STACK declares stack not executable
With rev 13368, Valgrind obeys PT_GNU_STACK making the stack not
executable. This makes inferior function call with GDB >= 7.5 failing,
as GDB places a breakpoint on the stack, which must be decoded
and translated by Valgrind to have the inferior function call properly done.
=> introduce a special case in the conditions to allow translation
when a segment is not executable but is readable and there is a
breakpoint at the address.


git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13381 a5019735-40e9-0310-863c-91ae7b9d1cf9