Only include stdint.h in __STDC_HOSTED__ (i.e. not -ffreestanding)

[kubamracek]

Attempt to fix this CI job on rebranch: https://ci.swift.org/view/Swift%20rebranch/job/oss-swift-rebranch-RA-linux-ubuntu-22_04-long-test/2833/console

In file included from /home/build-user/swift/stdlib/public/stubs/Unicode/UnicodeWord.cpp:14:
In file included from /home/build-user/swift/stdlib/public/stubs/Unicode/Common/WordData.h:19:
In file included from /home/build-user/swift/stdlib/public/SwiftShims/swift/shims/SwiftStdint.h:28:
In file included from /home/build-user/build/buildbot_incremental/llvm-linux-x86_64/bin/../include/c++/v1/stdint.h:106:
/home/build-user/build/buildbot_incremental/llvm-linux-x86_64/bin/../include/c++/v1/__config:924:8: error: "No thread API"
  924 | #      error "No thread API"
      |        ^
1 error generated.