feat: initial support for riscv64 (#2366)

* feat: initial support for riscv64

* use enable for riscv64

* Apply review suggestion

Co-authored-by: Joe Rickerby <joerick@mac.com>

* update documentation

---------

Co-authored-by: Joe Rickerby <joerick@mac.com>

cibuildwheel/selector.py

@@ -32,6 +32,7 @@ class EnableGroup(StrEnum):
     CPythonFreeThreading = "cpython-freethreading"
     CPythonPrerelease = "cpython-prerelease"
     PyPy = "pypy"
+    CPythonExperimentalRiscV64 = "cpython-experimental-riscv64"
 
     @classmethod
     def all_groups(cls) -> frozenset["EnableGroup"]:
@@ -70,6 +71,10 @@ class BuildSelector:
             return False
         if EnableGroup.PyPy not in self.enable and fnmatch(build_id, "pp*"):
             return False
+        if EnableGroup.CPythonExperimentalRiscV64 not in self.enable and fnmatch(
+            build_id, "*_riscv64"
+        ):
+            return False
 
         should_build = selector_matches(self.build_config, build_id)
         should_skip = selector_matches(self.skip_config, build_id)