Kernel: Shrink default userspace stack size from 4 MiB to 1 MiB

This knocks 70 MiB off our idle footprint, (from 350 MiB to 280 MiB.)

Kernel/API/Syscall.h

@@ -347,7 +347,7 @@ struct SC_create_thread_params {
     // ... ok, if you say so posix. Guess we get to lie to people about guard page size
     unsigned int guard_page_size = 0;          // Rounded up to PAGE_SIZE
     unsigned int reported_guard_page_size = 0; // The lie we tell callers
-    unsigned int stack_size = 4 * MiB;         // Default PTHREAD_STACK_MIN
+    unsigned int stack_size = 1 * MiB;         // Default PTHREAD_STACK_MIN
     void* stack_location;                      // nullptr means any, o.w. process virtual address
 #    if ARCH(X86_64)
     FlatPtr rdi;

Kernel/Thread.h

@@ -1060,7 +1060,7 @@ public:
     static IterationDecision for_each(Callback);
 
     static constexpr u32 default_kernel_stack_size = 65536;
-    static constexpr u32 default_userspace_stack_size = 4 * MiB;
+    static constexpr u32 default_userspace_stack_size = 1 * MiB;
 
     u64 time_in_user() const { return m_total_time_scheduled_user; }
     u64 time_in_kernel() const { return m_total_time_scheduled_kernel; }