[asan] Override default sigaltstack setting on Android.

Android libc always sets altstack on non-main threads, but it is too small for
our unwinder. With use_sigaltstack=1, override this setting with a larger one.


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@201557 91177308-0d34-0410-b5e6-96231b3b80d8