Fix Bullet physics build warnings

[madmiraal]

Currently, when building on macOS 15 with Clang 17, we are getting the following warnings, which we treat as errors:

  In file included from modules/bullet/bullet_physics_server.cpp:7:
  In file included from modules/bullet/bullet_physics_server.h:10:
  In file included from modules/bullet/area_bullet.h:10:
  In file included from modules/bullet/collision_object_bullet.h:16:
  In file included from thirdparty/bullet/LinearMath/btTransform.h:18:
  In file included from thirdparty/bullet/LinearMath/btMatrix3x3.h:18:
  thirdparty/bullet/LinearMath/btVector3.h:1096:2: error: definition of implicit copy constructor for 'btVector4' is deprecated because it has a user-provided copy assignment operator [-Werror,-Wdeprecated-copy-with-user-provided-copy]
   1096 |         operator=(const btVector4& v)
        |         ^
  thirdparty/bullet/LinearMath/btVector3.h:1106:10: note: in implicit copy constructor for 'btVector4' first required here
   1106 |                 return btVector4(_mm_and_ps(mVec128, btvAbsfMask));
        |                        ^

  In file included from modules/bullet/bullet_physics_server.cpp:7:
  In file included from modules/bullet/bullet_physics_server.h:10:
  In file included from modules/bullet/area_bullet.h:13:
  In file included from modules/bullet/space_bullet.h:12:
  In file included from modules/bullet/result_callbacks.h:13:
  In file included from thirdparty/bullet/btBulletDynamicsCommon.h:27:
  In file included from thirdparty/bullet/BulletDynamics/ConstraintSolver/btPoint2PointConstraint.h:21:
  In file included from thirdparty/bullet/BulletDynamics/ConstraintSolver/btTypedConstraint.h:20:
  In file included from thirdparty/bullet/BulletDynamics/ConstraintSolver/btSolverConstraint.h:26:
  thirdparty/bullet/BulletDynamics/ConstraintSolver/btSolverBody.h:59:20: error: 'const' type qualifier on return type has no effect [-Werror,-Wignored-qualifiers]
     59 |         SIMD_FORCE_INLINE const __m128 get128() const
        |                           ^~~~~

This PR applies the fixes sent upstream:

• Create explicit copy constructor for btVector4 bulletphysics/bullet3#4762: Create explicit copy constructor for btVector4.
• Create single const get128() method in btSimdScalar bulletphysics/bullet3#4763: Create single const get128() method in btSimdScalar.