[asan] fix two off-by-one errors that seem to affect only PowerPC because only...

[asan] fix two off-by-one errors that seem to affect only PowerPC because only there the stack top may be equal to the address space top. Noted by Andreas Schwab in http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55975#c11 . Also make swapcontext interceptor a bit more robust

llvm-svn: 172807