Revert "[libc] disable process_mrelease for riscv (llvm#117956)"

This reverts commit 700d9ac.

Author: SchrodingerZhu

libc/config/linux/riscv/entrypoints.txt

@@ -251,8 +251,7 @@ set(TARGET_LIBC_ENTRYPOINTS
     libc.src.sys.mman.munmap
     libc.src.sys.mman.remap_file_pages
     libc.src.sys.mman.posix_madvise
-    # TODO: disabled due to buildbot failure. further investigation needed.
-    # libc.src.sys.mman.process_mrelease
+    libc.src.sys.mman.process_mrelease
     libc.src.sys.mman.shm_open
     libc.src.sys.mman.shm_unlink