[libc] Disable '_exit' on the GPU build

Summary:
There are other dependencies to enable `unistd.h` on the GPU which
prevented the header from being generated. This is a POSIX extension and
isn't part of the core `libc`, so we can just disable this for now to
get the bots gree.

libc/config/gpu/entrypoints.txt

@@ -211,9 +211,6 @@ set(TARGET_LIBC_ENTRYPOINTS
 
     # gpu/rpc.h entrypoints
     libc.src.gpu.rpc_host_call
-
-    # unistd.h entrypoints
-    libc.src.unistd._exit
 )
 
 set(TARGET_LIBM_ENTRYPOINTS

libc/config/gpu/headers.txt

@@ -12,7 +12,6 @@ set(TARGET_PUBLIC_HEADERS
     libc.include.errno
     libc.include.stdlib
     libc.include.stdio
-    libc.include.unistd
 
     # Header for RPC extensions
     libc.include.gpu_rpc