[stdlib] Include POSIXError as part of Glibc

[colemancda]

POSIXErrorCode is compiled as part of the platform module on Darwin, but not as part of Glibc on Linux and other platforms. This PR makes the platforms consistently define POSIXErrorCode which should have been compiling for Linux since 8157194#diff-c86969eead9d9f63ad0faa3e75802e66R268.

Resolves SR-10485.

[compnerd]

@swift-ci please test

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 787bc36

[colemancda]

@compnerd Builds fine on Linux, not sure what the issue is with macOS.

18:45:51 ERROR: Step ‘Publish xUnit test result report’ failed: [JUnit] - No test report file(s) were found with the pattern 'branch-master/buildbot_incremental/swift-macosx-x86_64/swift-test-results/**/*.xml' relative to '/Users/buildnode/jenkins/workspace/swift-PR-osx' for the testing framework 'JUnit'.
18:45:51 Did you enter a pattern relative to (and within) the workspace directory?
18:45:51 Did you generate the result report(s) for 'JUnit'?"
18:45:51 [BFA] Scanning build for known causes...
18:45:51 [BFA] No failure causes found
18:45:51 [BFA] Done. 0s
18:45:51 Adding one-line test results to commit status...
18:45:51 Setting status of 787bc36f6d0b68c95052ce70ab90fa7145c4fabc to FAILURE with url https://ci.swift.org/job/swift-PR-osx/12537/ and message: 'Build finished. No test results found.'
18:45:51 Using context: Swift Test OS X Platform
18:45:51 **Build failed**

[compnerd]

@swift-ci please test macOS platform