Builds on the source-built-RTL link path. Two correctness fixes plus the
fixpoint-script migration that takes the self-host pipeline off blaise_rtl.a.
1. Symbol-based dedup. A program that uses an RTL unit (e.g. `uses classes`
pulls runtime.arc) compiles that unit into its own object cache, so
EnsureRTLObjects must not supply a second copy or the link double-defines it.
The dedup now compares the DEFINED symbols of the caller's objects against
each RTL object (via the ELF reader) and skips one only when fully redundant
— matching the old archive's member selection. An earlier filename-only
check wrongly suppressed the real bare-symbol provider when a same-named but
differently-mangled stale object was present.
2. Race-free build. EnsureRTLObjects now builds into a per-process private
directory (rtl/build-<pid>/) and atomically renames each finished object
into the shared cache. Parallel builders (e.g. e2e suites warming a cold
cache) no longer corrupt each other: a reader never sees a half-written
object, and two builders just publish identical results. The private dir is
cleaned up afterwards. getpid/rmdir are declared directly rather than via
GRtlPlatform to avoid a unit-init-order crash in the driver.
3. Fixpoint scripts off the archive. New scripts/build-rtl-objects.sh builds
the RTL from source (the script-level equivalent of EnsureRTLObjects), with
--exclude-defined-by so a whole-program --emit-ir/--emit-asm dump (which
inlines the RTL units the compiler uses) is not handed a duplicate copy.
fixpoint.sh and fixpoint-native.sh link via this helper instead of
blaise_rtl.a; fixpoint-native-internal.sh and fixpoint-warmcache.sh no longer
copy the archive beside the compiler (the driver source-builds the RTL).
All four fixpoints green (FIXPOINT_OK, NATIVE_FIXPOINT_OK, NATIVE_INTERNAL_OK,
WARMCACHE_FIXPOINT_OK); full suite green (3829 tests) under QBE- and
native-built runners; four cold-cache parallel suite runs pass.
The runtime Makefile / ar / make install are still present; removing them is the
final Stage-3 step.
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