Swift: remove linux from standard pack

This still defines a pack with linux included for development.
`//swift:install` will still also include linux.

misc/bazel/utils.bzl

@@ -0,0 +1,18 @@
+def select_os(linux = None, macos = None, windows = None, posix = None, otherwise = []):
+    selection = {}
+    if posix != None:
+        if linux != None or macos != None:
+            fail("select_os: cannot specify both posix and linux or macos")
+        selection["@platforms//os:linux"] = posix
+        selection["@platforms//os:macos"] = posix
+    if linux != None:
+        selection["@platforms//os:linux"] = linux
+    if macos != None:
+        selection["@platforms//os:macos"] = macos
+    if windows != None:
+        selection["@platforms//os:windows"] = windows
+    if len(selection) < 3:
+        selection["//conditions:default"] = otherwise
+    elif otherwise != []:
+        fail("select_os: cannot specify all three OSes and an otherwise")
+    return select(selection)