[common] Replace Common::SpinLock with std::mutex (#4436)

`std::mutex` can be better than the spin-lock specially since we don't waste CPU cycles for no reason.

Test less stuttering/heat consumption.

Signed-off-by: lizzie <lizzie@eden-emu.dev>

- [x] I have read and followed the [Contribution Guidelines](https://git.eden-emu.dev/eden-emu/eden/src/branch/master/CONTRIBUTING.md#code-contributions).
- [x] I have read and followed the [AI Policy](https://git.eden-emu.dev/eden-emu/eden/src/branch/master/docs/policies/AI.md)
- [x] I have read and followed the [Coding Guidelines](https://git.eden-emu.dev/eden-emu/eden/src/branch/master/docs/policies/Coding.md) to the best of my ability.

-------------------

Reviewed-on: https://git.eden-emu.dev/eden-emu/eden/pulls/4436
Reviewed-by: CamilleLaVey <camillelavey99@gmail.com>
Reviewed-by: MaranBr <maranbr@eden-emu.dev>

src/core/hle/kernel/k_thread.h

@@ -19,7 +19,6 @@
 
 #include "common/intrusive_red_black_tree.h"
 #include "common/scratch_buffer.h"
-#include "common/spin_lock.h"
 #include "core/arm/arm_interface.h"
 #include "core/hle/kernel/k_affinity_mask.h"
 #include "core/hle/kernel/k_light_lock.h"
@@ -920,7 +919,7 @@ private:
     bool m_resource_limit_release_hint{};
     bool m_is_kernel_address_key{};
     StackParameters m_stack_parameters{};
-    Common::SpinLock m_context_guard{};
+    std::mutex m_context_guard{};
 
     // For emulation
     std::shared_ptr<Common::Fiber> m_host_context{};