Add lit feature for stdlib variants

Author: Azoy

stdlib/public/core/Algorithm.swift

@@ -160,6 +160,4 @@ extension EnumeratedSequence: Sequence {
 }
 
 @available(SwiftStdlib 5.11, *)
-public func hello() {
-  print("hello!")
-}
+public func hello() {}

test/abi/macOS/arm64/stdlib.swift

@@ -7,11 +7,7 @@
 // RUN: diff -u %S/../../Inputs/macOS/arm64/stdlib/baseline %t/symbols
 
 // REQUIRES: swift_stdlib_no_asserts
-// REQUIRES: OS=macosx
-
-// FIXME: This isn't technically required. We really want a STDLIB_ARCH=arm64
-// feature that tells us we've built an arm64 version of the standard library.
-// REQUIRES: CPU=arm64
+// REQUIRES: STDLIB_VARIANT=macosx-arm64
 
 // *** DO NOT DISABLE OR XFAIL THIS TEST. *** (See comment below.)
 

test/abi/macOS/x86_64/stdlib.swift

@@ -7,11 +7,7 @@
 // RUN: diff -u %S/../../Inputs/macOS/x86_64/stdlib/baseline %t/symbols
 
 // REQUIRES: swift_stdlib_no_asserts
-// REQUIRES: OS=macosx
-
-// FIXME: This isn't technically required. We really want a STDLIB_ARCH=x86_64
-// feature that tells us we've built an x86_64 version of the standard library.
-// REQUIRES: CPU=x86_64
+// REQUIRES: STDLIB_VARIANT=macosx-x86_64
 
 // *** DO NOT DISABLE OR XFAIL THIS TEST. *** (See comment below.)
 

test/lit.site.cfg.in

@@ -73,6 +73,8 @@ config.testing_against_host_compiler = \
 
 # Please remember to handle empty strings and/or unset variables correctly.
 
+config.available_features.add("STDLIB_VARIANT={}".format(config.variant_suffix[1:]))
+
 if "@SWIFT_ASAN_BUILD@" == "TRUE":
     config.available_features.add("asan")
 else: