Move the OS integer constants (O_*/S_*/SEEK_*/CLOCK_REALTIME/WNOHANG) and the
struct stat layout out of rtl.platform.posix and behind a new TPlatformLayout
port. struct stat is now an opaque sized buffer whose Size/Mtime/Mode fields are
read at target offsets via StatSize/StatMtime/StatMode, rather than a record
that bakes in one layout — FreeBSD's stat differs in both offsets and size.
The Linux adapter (rtl.platform.layout.linux, TPlatformLayoutLinuxX86_64) supplies
today's values verbatim; struct stat offsets verified against sys/stat.h
(st_mode=24, st_size=48, st_mtim=88, sizeof=144). struct tm is left shared — its
used fields are identical on Linux/FreeBSD amd64.
The POSIX unit's one per-target wire is its 'uses rtl.platform.layout.linux';
the FreeBSD RTL archive will compose its own layout. No conditional compilation.
Step 0b of docs/native-target-architecture.adoc. Behaviour-preserving:
FIXPOINT_OK, NATIVE_FIXPOINT_OK, NATIVE_INTERNAL_OK, WARMCACHE_FIXPOINT_OK;
full suite 3761 tests green; file I/O (FileExists/DirectoryExists/FileAge/
ReadFile) exercised end-to-end through the new accessors.
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