[GR-80028] Remove PosixSupportLibrary.

[graalvmbot]

Replace PosixSupportLibrary with ordinary Java methods and inheritance.

Following the migration from Truffle NFI to FFM, the POSIX backend implementations gain little from Truffle partial evaluation. Path and C-string conversions still benefit from cached TruffleString nodes, so this change moves those conversions into PosixSupportNodes at the call sites and executes backend operations behind @TruffleBoundary.
This reduces the amount of POSIX implementation code reachable for runtime compilation while keeping string conversions visible to partial evaluation.

In detail:

1. NativePosixSupport represents paths and C strings as encoded bytes, stored in byte[] and copied to off-heap memory for native calls. Windows paths use UTF-16LE for the wide APIs.
2. EmulatedPosixSupport uses java.lang.String.
3. PosixSupportNodes converts between TruffleString and these backend representations.
4. POSIX operations use ordinary Java methods annotated with @TruffleBoundary.