Make sure the stack pointer is properly aligned when invoking a

Make sure the stack pointer is properly aligned when invoking a
signal on amd64-linux systems.

The amd64 ABI describes the required alignment on function entry
as follows:

  "In other words, the value (%rsp − 8) is always a multiple
   of 16 when control is transferred to the function entry point.

So we need to 16 byte align and then subtract an extra 8 bytes
to achieve the correct alignment.

Patch from fjgmacc@gmail.com to fix BZ#280114.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13182