Host-side static ELF checks (EI_OSABI/ET_EXEC/no-PT_INTERP, already asserted by
TInternalLinkerE2ETests.TestCompile_FreeBSDTarget_EmitsStaticFreeBSDExe) cannot
catch a wrong FreeBSD struct-stat offset or syscall number — only running the
binary on a real FreeBSD kernel does.
Add a freebsd-crosscheck CI job (needs: bootstrap) that:
* downloads the -pre binary the bootstrap job built,
* cross-compiles the fixtures in compiler/src/it/freebsd (hello, fileio) with
--target freebsd-x86_64 on the Linux runner via
scripts/freebsd-crosscheck-build.sh (which also sanity-checks each emitted
ELF is a FreeBSD ET_EXEC), and
* boots a FreeBSD VM (vmactions/freebsd-vm, release 14.2 to match the
struct-stat offsets in rtl.platform.layout.freebsd) and runs the binaries
there, asserting hello prints 'Hello' and fileio round-trips a file
(write/read/stat/delete) with 'file-io-ok'/'exists-ok'/'done'.
The FreeBSD output is a static, freestanding raw-syscall ET_EXEC, so the VM
needs no extra packages to run it. fileio is the meaningful check: it exercises
open/write/read/stat/unlink through the FreeBSD leaf and layout, which the
static checks are blind to.
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