feat: add FreeBSD pthread support to NIOLock

[max-potapov]

Motivation

NIOLock's pthread fallback branch types LockPrimitive as
pthread_mutex_t and initializes pthread_mutexattr_t with a default
initializer; both compile on Linux but fail on FreeBSD / OpenBSD where
those types are opaque-pointer typedefs imported as
Optional<OpaquePointer>. Additionally, PTHREAD_MUTEX_ERRORCHECK is
imported as a pthread_mutextype enum case on FreeBSD whose .rawValue
must be used to construct the CInt that pthread_mutexattr_settype
expects.

This blocks swift-prometheus from compiling on FreeBSD with the
official Swift FreeBSD preview toolchain.

Modifications

Mirror the pattern landed in apple/swift-log#387; same fix is
being submitted for the sibling NIOLock copies in
apple/swift-http-types#123 and
swift-server/swift-service-lifecycle:

• On os(FreeBSD) || os(OpenBSD) alias LockPrimitive to
  pthread_mutex_t?.
• Initialize pthread_mutexattr_t via the bitPattern: overload.
• On os(FreeBSD) || os(OpenBSD), access PTHREAD_MUTEX_ERRORCHECK
  via .rawValue before passing to pthread_mutexattr_settype.

Touches one file (Sources/Prometheus/NIOLock.swift), +11 lines, all
behind FreeBSD/OpenBSD conditionals. No behaviour change on existing
platforms.

Result

swift build and swift test both clean on FreeBSD 15.0 with the
official Swift FreeBSD preview toolchain:

$ uname -a
FreeBSD swiftbuild 15.0-RELEASE FreeBSD 15.0-RELEASE-p9 releng/15.0-n281048-6d536196f1bd GENERIC amd64

$ swift --version
Swift version 6.3-dev (LLVM 972b62858355d07, Swift 3f8c798cfe25bbb)
Target: x86_64-unknown-freebsd14.3
Build config: +assertions

$ swift build
... Compiling Prometheus NIOLock.swift ...
Build complete! (15.77s)

$ swift test
... 40 tests, with 0 failures (0 unexpected) ...
✔ Test run passed.

Without this change Prometheus fails to compile on the same
toolchain.

Checks

• Builds on Linux/Darwin (no change there, all changes guarded
  behind #if os(FreeBSD) || os(OpenBSD)).
• Builds + passes existing 40 tests on FreeBSD 15.0 with Swift
  6.3-dev preview toolchain.
• Same pattern as apple/swift-log#387 /
  apple/swift-http-types#123.
• Commit title follows the project's Conventional Commits
  convention (feat: ...).