[Runtimes][CMake] Rename SwiftCore target to swiftCore

[edymtt]

This matches the case that we use in the Runtimes build system, so we rely the exported target instead of adding -lswiftCore.

Addresses rdar://151700669

[edymtt]

@swift-ci please smoke test

[edymtt]

macOS smoke test failed because in the Runtime build system we are using Swift.swiftmodule from the SDK instead of the one we just build

2025-05-21T16:46:48.767Z] /Users/ec2-user/jenkins/workspace/swift-PR-macos-smoke-test/branch-main/swift/Runtimes/Supplemental/StringProcessing/_StringProcessing/Unicode/NFC.swift:33:32: error: value of type 'String' has no member '_nfc'
[2025-05-21T16:46:48.767Z] 31 |     // SwiftStdlib is always >= 5.7 for a shipped StringProcessing.
[2025-05-21T16:46:48.767Z] 32 |     guard #available(SwiftStdlib 5.7, *) else { return nil }
[2025-05-21T16:46:48.767Z] 33 |     var nfcIter = String(self)._nfc.makeIterator()
[2025-05-21T16:46:48.767Z]    |                                `- error: value of type 'String' has no member '_nfc'
[2025-05-21T16:46:48.767Z] 34 |     guard let scalar = nfcIter.next(), nfcIter.next() == nil else { return nil }
[2025-05-21T16:46:48.767Z] 35 |     return scalar
[2025-05-21T16:46:48.767Z] /Users/ec2-user/jenkins/workspace/swift-PR-macos-smoke-test/branch-main/swift/Runtimes/Supplemental/StringProcessing/_StringProcessing/Unicode/WordBreaking.swift:14:2: warning: '@_spi' import of 'Swift' will not include any SPI symbols; 'Swift' was built from the public interface at /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX15.2.sdk/usr/lib/swift/Swift.swiftmodule/arm64e-apple-macos.swiftinterface

Amending this in #81687

[edymtt]

@swift-ci please smoke test