[libc] Add atexit to baremetal entrypoints (llvm#147290)

Part of llvm#145349. Requires llvm#145358. Required by llvm#146863. Once the mutex
has been implemented, we can register functions to be called for exit
with `atexit`.

Author: saturn691

libc/config/baremetal/aarch64/entrypoints.txt

@@ -218,6 +218,7 @@ set(TARGET_LIBC_ENTRYPOINTS
     libc.src.stdlib.abort
     libc.src.stdlib.abs
     libc.src.stdlib.aligned_alloc
+    libc.src.stdlib.atexit
     libc.src.stdlib.atof
     libc.src.stdlib.atoi
     libc.src.stdlib.atol

libc/config/baremetal/arm/entrypoints.txt

@@ -218,6 +218,7 @@ set(TARGET_LIBC_ENTRYPOINTS
     libc.src.stdlib.abort
     libc.src.stdlib.abs
     libc.src.stdlib.aligned_alloc
+    libc.src.stdlib.atexit
     libc.src.stdlib.atof
     libc.src.stdlib.atoi
     libc.src.stdlib.atol

libc/config/baremetal/riscv/entrypoints.txt

@@ -218,6 +218,7 @@ set(TARGET_LIBC_ENTRYPOINTS
     libc.src.stdlib.abort
     libc.src.stdlib.abs
     libc.src.stdlib.aligned_alloc
+    libc.src.stdlib.atexit
     libc.src.stdlib.atof
     libc.src.stdlib.atoi
     libc.src.stdlib.atol