Add x86_64-unknown-none-elf to platform-support.md.

Author: mikeleany

src/doc/rustc/src/platform-support.md

@@ -280,6 +280,7 @@ target | std | host | notes
 `x86_64-unknown-haiku` | ✓ | ✓ | 64-bit Haiku
 `x86_64-unknown-hermit` | ? |  |
 `x86_64-unknown-l4re-uclibc` | ? |  |
+`x86_64-unknown-none-elf` | * |  | Bare x86_64, softfloat
 `x86_64-unknown-none-hermitkernel` | ? |  | HermitCore kernel
 `x86_64-unknown-none-linuxkernel` | * |  | Linux kernel modules
 `x86_64-unknown-openbsd` | ✓ | ✓ | 64-bit OpenBSD