[Backtracing][Linux] Properly align the stacks.

We have two stacks that we set up during crash handling, both of
which were potentially misaligned.  This only tripped us up on
Ubuntu 20.04 on aarch64 - my guess is that maybe `clone()` aligns
the stack pointer on newer kernels which is why we didn't see it
on 22.04.

rdar://110743884

Author: al45tair

stdlib/public/runtime/Backtrace.cpp

@@ -796,7 +796,7 @@ struct spawn_info {
   int memserver;
 };
 
-uint8_t spawn_stack[4096];
+uint8_t spawn_stack[4096] __attribute__((aligned(SWIFT_PAGE_SIZE)));
 
 int
 do_spawn(void *ptr) {

stdlib/public/runtime/CrashHandlerLinux.cpp

@@ -607,7 +607,7 @@ wait_paused(uint32_t expected, const struct timespec *timeout)
    We don't want to require CAP_SYS_PTRACE because we're potentially being
    used inside of a Docker container, which won't have that enabled. */
 
-char memserver_stack[4096];
+char memserver_stack[4096] __attribute__((aligned(SWIFT_PAGE_SIZE)));
 char memserver_buffer[4096];
 int memserver_fd;
 bool memserver_has_ptrace;