Fix watchOS and visionOS for pread64 and pwrite64 calls
In #122880, links to `preadv64` and `pwritev64` were added for `watchOS` however the underlying [`weak!` macro did not include `target_os = "watchos"`](c45dee5efd/library/std/src/sys/pal/unix/weak.rs (L30-L74)).
This resulted in an `xcodebuild` error when targeting `watchOS`:
```
Undefined symbols for architecture arm64:
"_preadv64", referenced from:
__rust_extern_with_linkage_preadv64 in libliveview_native_core.a[274](std-324fdd8d31e8eaa2.std.e18cf7e8d0336778-cgu.08.rcgu.o)
"_pwritev64", referenced from:
__rust_extern_with_linkage_pwritev64 in libliveview_native_core.a[274](std-324fdd8d31e8eaa2.std.e18cf7e8d0336778-cgu.08.rcgu.o)
ld: symbol(s) not found for architecture arm64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
```
So I added them. I also went ahead and added the same for visionOS because it's bound to create the same issue.
9efd1477ac