linx: extend loader bring-up for shared libc

[zhoubot]

Summary

• add the Linx startup entry and Linux/Linx stat+mmap loader/syscall glue needed for shared glibc bring-up
• relax the Linx rtld segment-mapping path so shared libc can load in the current runtime environment
• keep the existing Linx rtld bring-up tracing around file-id, versioning, and profiling decisions

Validation

• cd /Users/zhoubot/linx-isa && GLIBC_G1B_ALLOW_BLOCKED=1 bash lib/glibc/tools/linx/build_linx64_glibc_g1b.sh
  • status: pass
  • classification: shared_libc_so_built
  • artifact: out/libc/glibc/build/linkobj/libc.so