kbuild, rust: use -fremap-path-prefix to make paths relative

Remap source path prefixes in all output, including compiler
diagnostics, debug information, macro expansions, etc.
This removes a few absolute paths from the binary and also makes it
possible to use core::panic::Location properly.

Equivalent to the same configuration done for C sources in
commit 1d3730f0012f ("kbuild: support -fmacro-prefix-map for external
modules") and commit a73619a845d5 ("kbuild: use -fmacro-prefix-map to
make __FILE__ a relative path").

Link: https://doc.rust-lang.org/rustc/command-line-arguments.html#--remap-path-prefix-remap-source-names-in-output
Acked-by: Miguel Ojeda <ojeda@kernel.org>
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Tested-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>