Add mach_error_string (and mach_error_t) (rust-lang#3913)

`mach_error_string` is defined in `/usr/include/mach/mach_error.h`
It is not referenced in the documentation Apple website.

```
char            *mach_error_string(
/*
 *  Returns a string appropriate to the error argument given
 */
    mach_error_t error_value
    );
```

`mach_error_t` is defined in `/usr/include/mach/error.h`
https://developer.apple.com/documentation/kernel/mach_error_t

```
typedef kern_return_t   mach_error_t;
```

(backport <rust-lang#3913>)
(cherry picked from commit d8ff07b)

Author: stepancheg

libc-test/semver/apple.txt

@@ -2020,6 +2020,8 @@ log2phys
 login_tty
 lutimes
 mach_absolute_time
+mach_error_string
+mach_error_t
 mach_header
 mach_header_64
 mach_host_self

src/unix/bsd/apple/mod.rs

@@ -56,6 +56,7 @@ pub type thread_inspect_t = ::mach_port_t;
 pub type thread_act_t = ::mach_port_t;
 pub type thread_act_array_t = *mut ::thread_act_t;
 pub type policy_t = ::c_int;
+pub type mach_error_t = ::kern_return_t;
 pub type mach_vm_address_t = u64;
 pub type mach_vm_offset_t = u64;
 pub type mach_vm_size_t = u64;
@@ -6330,6 +6331,8 @@ extern "C" {
     pub fn copyfile_state_get(s: copyfile_state_t, flags: u32, dst: *mut ::c_void) -> ::c_int;
     pub fn copyfile_state_set(s: copyfile_state_t, flags: u32, src: *const ::c_void) -> ::c_int;
 
+    pub fn mach_error_string(error_value: ::mach_error_t) -> *mut ::c_char;
+
     // Added in macOS 10.13
     // ISO/IEC 9899:2011 ("ISO C11") K.3.7.4.1
     pub fn memset_s(s: *mut ::c_void, smax: ::size_t, c: ::c_int, n: ::size_t) -> ::c_int;