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

libc/bionic/pthread_internal.h

@@ -167,9 +167,14 @@ __LIBC_HIDDEN__ void pthread_key_clean_all(void);
 
 // SIGSTKSZ (8KiB) is not big enough.
 // An snprintf to a stack buffer of size PATH_MAX consumes ~7KiB of stack.
-// Also, on 64-bit, logging uses more than 8KiB by itself:
-// https://code.google.com/p/android/issues/detail?id=187064
+// On 64-bit, logging uses more than 8KiB by itself, ucontext is comically
+// large on aarch64, and we have effectively infinite address space, so double
+// the signal stack size.
+#if defined(__LP64__)
+#define SIGNAL_STACK_SIZE_WITHOUT_GUARD (32 * 1024)
+#else
 #define SIGNAL_STACK_SIZE_WITHOUT_GUARD (16 * 1024)
+#endif
 
 // Traditionally we gave threads a 1MiB stack. When we started
 // allocating per-thread alternate signal stacks to ease debugging of