kernel: dynamic: use 4k stack size for x86

x86 architectures require a dynamic stack size that is a multiple
of 4096 bytes due to mmu restrictions.

For example, this test would previously fail when using the
default dynamic stack size of 1024 bytes for 32-bit
platforms.

```
west build -p auto -b qemu_x86/atom/nopae -t run \
  tests/posix/common/ -- -DCONFIG_USERSPACE=y
```

It would pass with an additional argument
```
west build -p auto -b qemu_x86/atom/nopae -t run \
  tests/posix/common/ -- -DCONFIG_USERSPACE=y \
  -DCONFIG_DYNAMIC_THREAD_STACK_SIZE=4096
```

Add a special default for x86 when using dynamic thread stacks.

The x86 default removes the need for `boards/qemu_x86*.conf`,
with the exception of `qemu_x86_tiny`.

qemu_x86_tiny did not have sufficient memory (or configuration)
to run the non-userspace tests, so bump up the available ram
from 256k to 512k for this test and clone the .conf from the
demand paging tests.

Eventually, the common posix test should be split into more
concise functional categories.

Signed-off-by: Chris Friedt <[email protected]>

kernel/Kconfig

@@ -245,8 +245,9 @@ if DYNAMIC_THREAD
 
 config DYNAMIC_THREAD_STACK_SIZE
 	int "Size of each pre-allocated thread stack"
-	default 1024 if !64BIT
-	default 2048 if 64BIT
+	default 4096 if X86
+	default 1024 if !X86 && !64BIT
+	default 2048 if !X86 && 64BIT
 	help
 	  Default stack size (in bytes) for dynamic threads.
 

tests/posix/common/boards/qemu_x86_tiny.conf

@@ -0,0 +1,16 @@
+# Copyright (c) 2021 Intel Corporation
+# SPDX-License-Identifier: Apache-2.0
+
+# The test is highly sensitive to size of kernel image.
+# However, specifying how many pages used by
+# the backing store must be done in build time.
+# So here we are, tuning this manually.
+CONFIG_BACKING_STORE_RAM_PAGES=12
+
+# The following is needed so that .text and following
+# sections are present in physical memory to test
+# using backing store for anonymous memory.
+CONFIG_KERNEL_VM_BASE=0x0
+CONFIG_LINKER_GENERIC_SECTIONS_PRESENT_AT_BOOT=y
+CONFIG_BACKING_STORE_RAM=y
+CONFIG_BACKING_STORE_QEMU_X86_TINY_FLASH=n

tests/posix/common/boards/qemu_x86_tiny.overlay

@@ -0,0 +1,11 @@
+/*
+ * Copyright (c) 2024, Tenstorrent AI ULC
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+/ {
+	dram0: memory@0 {
+		reg = <0x100000 0x80000>;
+	};
+};