Update platform-support.md

Merged in rust-lang/rust#56066

Author: jethrogb

platform-support.md

@@ -144,6 +144,7 @@ Official builds are not available.
 | `thumbv7em-none-eabihf`         |  *  |     |     | Bare Cortex-M4F, M7F, FPU, hardfloat                     |
 | `thumbv7m-none-eabi`            |  *  |     |     | Bare Cortex-M3                                           |
 | `x86_64-pc-windows-msvc` (XP)   |  ✓  |     |     | Windows XP support                                       |
+| `x86_64-fortanix-unknown-sgx`   |  ✓  |     |     | Fortanix ABI for 64-bit Intel SGX                        |
 | `x86_64-unknown-bitrig`         |  ✓  |  ✓  |     | 64-bit Bitrig                                            |
 | `x86_64-unknown-dragonfly`      |  ✓  |  ✓  |     | 64-bit DragonFlyBSD                                      |
 | `x86_64-unknown-haiku`          |  ✓  |     |     | 64-bit Haiku                                             |