Change the ABI CallFrameAddressIsValid methods for i386 and x86_64.
They were enforcing 16-byte alignment on stack frames for Darwin x86 programs.
But we've found that trap handlers typically don't have the stack pointer
aligned correctly when a trap happens and lldb wasn't backtracing all
the way through.  This method is only used as a safety guard to prevent
lldb's unwinder from using a bogus address as a stack frame - we'll still
enforce word-size alignment on stack frames so that should be fine.

Also rolled back akaylor's changes from August 2013 in r188952 which changed
the i386 ABI plugin to relax the CallFrameAddressIsValid offsets for non-Darwin
targets where only 4-byte alignment is enforced.  Now Darwin is the same as
those environments.

<rdar://problem/15982682>