[libc] Include _Exit in the baremetal configuration (#98053)

This is implemented using `__llvm_libc_exit` which should be provided by
the vendor and thus available in baremetal configurations.

Author: petrhosek

libc/config/baremetal/arm/entrypoints.txt

@@ -165,6 +165,7 @@ set(TARGET_LIBC_ENTRYPOINTS
     libc.src.stdbit.stdc_trailing_zeros_us
 
     # stdlib.h entrypoints
+    libc.src.stdlib._Exit
     libc.src.stdlib.abort
     libc.src.stdlib.abs
     libc.src.stdlib.aligned_alloc

libc/config/baremetal/riscv/entrypoints.txt

@@ -161,6 +161,7 @@ set(TARGET_LIBC_ENTRYPOINTS
     libc.src.stdbit.stdc_trailing_zeros_us
 
     # stdlib.h entrypoints
+    libc.src.stdlib._Exit
     libc.src.stdlib.abort
     libc.src.stdlib.abs
     libc.src.stdlib.aligned_alloc