[libc] Restore sysconf to default entrypoints (#197696)
Reverts the experimental gating of sysconf introduced in 1d93fc4f74fe
and restores it to the Scudo integration test entrypoints (undoing
8257855f77d5).
sysconf works correctly for the constants it handles. Moving it behind
the experimental flag broke the Scudo build since Scudo calls
sysconf(_SC_PAGESIZE).
Changes:
* libc/config/linux/{x86_64,aarch64,riscv}/entrypoints.txt: moved
sysconf back into TARGET_LIBC_ENTRYPOINTS, removed the
LLVM_LIBC_ENABLE_EXPERIMENTAL_ENTRYPOINTS guard.
* libc/test/integration/scudo/CMakeLists.txt: restored
libc.src.unistd.sysconf to the Scudo entrypoint library.
Assisted-by: Automated tooling, human reviewed.
