[AArch64][RISCV] Document option --print-supported-extensions

Change-Id: Ib0ff5deeb9415818ccd0480cfbcdadc97ddc9872

Author: jthackray

clang/docs/CommandGuide/clang.rst

@@ -400,6 +400,14 @@ number of cross compilers, or may only support a native target.
   option is only supported on AArch64 and RISC-V. On RISC-V, this option also
   prints out the ISA string of enabled extensions.
 
+.. option:: --print-supported-extensions
+
+  Prints the list of extensions that are supported for any possible CPU target
+  for the architecture specified by the combination of `--target`, `-march`,
+  and `-mcpu` values. Currently, this option is only supported on AArch64
+  and RISC-V. On RISC-V, this option also prints out the ISA string of
+  enabled extensions.
+
 Code Generation Options
 ~~~~~~~~~~~~~~~~~~~~~~~