[llvm] [RISCV][doc] Document profiles in RISCVUsage (PR #98326)

[Jessica Clarke via llvm-commits]

================
@@ -70,6 +70,33 @@ To specify the target triple:
 To select an E variant ISA (e.g. RV32E instead of RV32I), use the base
 architecture string (e.g. ``riscv32``) with the extension ``e``.
 
+Profiles
+========
+
+Supported profile names can be passed using ``-march`` instead of a standard
+ISA naming string. Currently supported profiles:
+
+* ``rvi20u32``
+* ``rvi20u64``
+* ``rva20u64``
+* ``rva20s64``
+* ``rva22u64``
+* ``rva22s64``
+
+Note that you can also append additional extension names to be enable, e.g.
+``rva20u64_zicond`` will enable the ``zicond`` extension in addition to those
+in the ``rva20u64`` profile.
+
+Profiles that are not yet ratified cannot be used unless
+``-menable-experimental-extensions`` (or equivalent for other tools) is
----------------
jrtc27 wrote:

Are there tools that accept ISA strings rather than target feature lists but don't call their options -march and -menable-experimental-extensions? I'm drawing a blank but maybe I'm missing something obvious...

https://github.com/llvm/llvm-project/pull/98326