Grow runtime.syscall.freebsd.pas from the Step-3 minimum (_exit/write) to
the file/process syscall leaves, mirroring runtime.syscall.linux but with
FreeBSD syscall numbers and FreeBSD's carry-flag error convention.
Added leaves: open close read lseek fstat stat mkdir rmdir unlink rename
chdir chmod getpid dup2 pipe mmap munmap nanosleep clock_gettime fork
execve wait4 kill getcwd. Each emits the raw `syscall` (number in %rax,
args rdi/rsi/rdx/r10/r8/r9, %rcx->%r10 for >=4 args) and translates
FreeBSD's error convention (CF set, positive errno in %rax) into the
-errno the Pascal call sites in rtl.platform.posix expect:
movq $N, %rax ; syscall ; jae .Lok ; negq %rax ; .Lok: ret
Syscall numbers are the FreeBSD 14 ino64 ABI (mmap=477 lseek=478
fstat=551, NOT the freebsd6/11 compat 197/199/189). FreeBSD's ino64 ABI
has no plain `stat` syscall, so stat(path,buf) is a wrapper over
fstatat(AT_FDCWD,path,buf,0)=552; pipe(fds) wraps pipe2(fds,0)=542. All
numbers verified against release/14.0.0 sys/sys/syscall.h.
The unit is standalone (only runtime.start.static.freebsd references it,
itself outside the Linux build graph), so the Linux self-host is
untouched. Test TestLink_FreeBSDSyscallLeaf_SymbolsAndNumbers links a
fixture and asserts the bare POSIX symbols are defined and the FreeBSD
syscall-number bytes + negq (CF xlate) + mov %rcx,%r10 are present.
Scope: file/process only; memcpy/date/allocator/threads are Step 4b/4c.
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