Increase the size of the signal stack on 64-bit.

The signal stack is sufficiently large for a single invocation of the
signal handler, but in cases where the signal handler needs to recurse,
(e.g. if our address space is limited by RLIMIT_AS), it's too small for
us to get to the part where we recognize that we've recursed and bail
out.

Bug: http://b/118772392
Test: /data/nativetest64/debuggerd_test/debuggerd_test64 --gtest_filter=CrasherTest.seccomp_crash_oom
Change-Id: Ic7a2cf8b01b3f7ea7f4a2318a3ec22a0c3649da6