rust: Upgrade to 1.56.0

This is latest major release, changes are here [1]

Forward port libstd-rs patches and refresh musl/rv64 port

[1] https://github.com/rust-lang/rust/releases/tag/1.56.0



Signed-off-by: Khem Raj <raj.khem@gmail.com>
Cc: Randy MacLeod <Randy.MacLeod@windriver.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>