Document --print-supported-extensions and --print-enabled-extensions

This is based on the implementation in Clang.

Author: lenary

src/toolchain-conventions.adoc

@@ -496,3 +496,20 @@ is platform defined.
 - `unlabeled`: Use simple label scheme, the label is always `0`.
 - `func-sig`: Use function signature as the label, the label is generated by the
   compiler, the rule is defined in psABI spec.
+
+===  `--print-supported-extensions` / `--print-enabled-extensions`
+
+These two flags are used to understand which RISC-V extensions are supported by
+a toolchain, and to understand exactly what extensions a given `-march=` or
+`-mcpu=` option expands to.
+
+The `--print-supported-extensions` option should print a list of extensions,
+with information about the extension ID, the supported version, and a short
+one-line description of that extension. LLVM separates this list into supported
+and experimental extensions. This output also includes a list of profile names
+supported by the compiler.
+
+The `--print-enabled-extension` option is intended to be used with `-march=` or
+`-mcpu=`, and prints only the extensions which are enabled for the current
+configuration, in the same format as `--print-supported-extensions`. This also
+prints the canonical architecture string for the given configuration.