Add VG_(am_is_bogus_client_stack_pointer)(Addr).
The function is used in VG_(client_syscall) to avoid extending the stack
when it is clear that the current value of the stack pointer does not
point into a segment that looks like a stack segment.
See the comments in the code there.
As a side effect of this we can now revert r15018 which increased
the stack size of the alternate stack in memcheck/tests/sigaltstack.c.
The reason is that the belief at the time: "alternate stack is too small"
was not correct. What instead happened was that VG_(client_syscall) called
VG_(extend_stack) without need (the syscall was tgkill) and the new stack
pointer happened to be in a file segment. 
In other words: the current stack pointer was still within the alternate
stack, i.e. the alternate stack was (barely) large enough.

git-svn-id: svn:// a5019735-40e9-0310-863c-91ae7b9d1cf9
5 files changed