In a 3-quart saucepan, cover the tomatillos and chiles with cold water and bring to a boil. Reduce the heat and simmer the vegetables uncovered, turning occasionally, until the tomatillos are tender and khaki-green all over but still intact, about 15 minutes.
Heat the cumin, allspice berries, and clove in a small, dry skillet over medium heat, shaking the pan or stirring the spices until they are fragrant and the cumin seeds are a few shades darker, about 1 minute.
Put the spices in a blender along with 1 cup of the tomatillo cooking water and blend until the spices are ground. Using a slotted spoon, gently lift the tomatillos and chiles out of the remaining cooking water and put them in the blender along with the garlic, cilantro, and 1⁄2 tsp. salt. Blend until smooth.
Pat the pork dry and season with the remaining 1 tsp. salt.
Heat the oil in a wide, heavy 4- to 5-quart pot over medium-high heat and brown the pork in batches without crowding, turning occasionally, about 8 minutes per batch.
Return all the meat to the pan and add the tomatillo sauce. Bring to a simmer, stirring to coat the meat, then reduce the heat. Simmer the pork, covered, stirring occasionally, until the meat is tender and the sauce is thickened, 1-1⁄2 to 2 hours. If necessary, continue to cook uncovered to thicken the sauce. Shred the meat with two forks.
Make the tacos with the tortillas, onion, cilantro, and crema.
Note: The pork in green sauce will keep chilled 3 days.