[stdlib] Adopt _pointerBitWidth conditional

[lorentey]

Replace arch(foo) || arch(bar) || arch(fred) incantations with the new _pointerBitWidth conditional.

[lorentey]

@swift-ci test

[stephentyrone]

Thanks for working on this one!

[lorentey]

arm64_32-apple-watchos4.0:

=== /Users/ec2-user/jenkins/workspace/swift-PR-macos/branch-main/swift/stdlib/public/core/StringObject.swift:171 ===
     ┆
 169 │ 
 170 │ #else
 171 │ #error("Unknown platform")
     │        ╰─ error: Unknown platform
 172 │ #endif
 173 │ 

[lorentey]

Oops, typo:

diff --git a/stdlib/public/core/StringObject.swift b/stdlib/public/core/StringObject.swift
index f3a440139b3..cd47c3b5313 100644
--- a/stdlib/public/core/StringObject.swift
+++ b/stdlib/public/core/StringObject.swift
@@ -101,7 +101,7 @@ internal struct _StringObject {
     self._object = Builtin.valueToBridgeObject(UInt64(0)._value)
   }
 
-#elseif _pointerBitWidth(_64)
+#elseif _pointerBitWidth(_32)
 
   @usableFromInline @frozen
   internal enum Variant {

I'll amend the existing commit.

[lorentey]

@swift-ci test

[lorentey]

The last commit normalized the ordering of the conditional clauses to 64 bit → 32 bit → error, even outside of StringObject. Putting the (far) more common case first is a nice didactic tool for avoiding confusion. (Thanks @glessard)

[lorentey]

@swift-ci test