Auto merge of #104101 - betrusted-io:xous-libstd-initial, r=bjorn3

Add initial libstd support for Xous

This patchset adds some minimal support to the tier-3 target `riscv32imac-unknown-xous-elf`. The following features are supported:

* alloc
* thread creation and joining
* thread sleeping
* thread_local
* panic_abort
* mutex
* condvar
* stdout

Additionally, internal support for the various Xous primitives surrounding IPC have been added as part of the Xous FFI. These may be exposed as part of `std::os::xous::ffi` in the future, however for now they are not public.

This represents the minimum viable product. A future patchset will add support for networking and filesystem support.

library/std/src/lib.rs

@@ -260,6 +260,7 @@
     feature(slice_index_methods, coerce_unsized, sgx_platform)
 )]
 #![cfg_attr(windows, feature(round_char_boundary))]
+#![cfg_attr(target_os = "xous", feature(slice_ptr_len))]
 //
 // Language features:
 // tidy-alphabetical-start