Add platform support doc for rv32e

Author: hegza

src/doc/rustc/src/platform-support/riscv32e-unknown-none-elf.md

@@ -0,0 +1,30 @@
+# `riscv32{e,em,emc}-unknown-none-elf`
+
+**Tier: 3**
+
+Bare-metal target for RISC-V CPUs with the RV32E, RV32EM and RV32EMC ISAs.
+
+## Target maintainers
+
+* Henri Lunnikivi, <henri.lunnikivi@gmail.com>, [@hegza](https://github.com/hegza)
+
+## Requirements
+
+The target is cross-compiled, and uses static linking. No external toolchain is
+required and the default `rust-lld` linker works, but you must specify a linker
+script.
+
+## Building the target
+
+This target is included in Rust and can be installed via `rustup`.
+
+## Testing
+
+This is a cross-compiled `no-std` target, which must be run either in a
+simulator or by programming them onto suitable hardware. It is not possible to
+run the Rust test-suite on this target.
+
+## Cross-compilation toolchains and C code
+
+This target supports C code. If interlinking with C or C++, you may need to use
+`riscv64-unknown-elf-gcc` as a linker instead of `rust-lld`.