Rework the extension test macros section

We have a list of macros, that should be extended for each new
extension. Let's rephrase the section such, that the wording
targets all extensions that are supported by compilers and
remove the incomplete list.

Signed-off-by: Christoph Müllner <[email protected]>

riscv-c-api.md

@@ -108,15 +108,25 @@ Perhaps obvious, but these are mutually exclusive, so only one is defined at a t
 for a compilation unit.
 
 
-### Architecture Extension Test Macro
+### Architecture Extension Test Macros
 
-Architecture extension test macro is a new set of test macro to checking the
-availability and version for certain extension, however not all compilers are
-supported, so you should check `__riscv_arch_test` to make sure this compiler
-is supporting those preprocessor definitions.
+Architecture extension test macros allow checking the availability and version
+of extensions at compile-time. These feature macros are optional, and compilers
+that support them provide a `__riscv_arch_test` macro (with the value `1`).
 
-The value of architecture extension test macro are defined as its version,
-which is compute by following formula:
+The naming rule for the test macros is `__riscv_<ext_name>`,
+where `<ext_name>` is all lower-case. Examples:
+- The test macro for the `A` extension is `__riscv_a`.
+- The test macro for the `Zifencei` extension is `__riscv_zifencei`.
+- The test macro for the `XVentanaCondOps` extension is `__riscv_xventantacondops`.
+
+Besides extensions, test macros also exist for ISA bases following the same pattern.
+Examples:
+- The test macro for the `I` base is `__riscv_i`.
+- The test macro for the `E` base is `__riscv_e`.
+
+The value of the test macros is derived from its version using the following
+formula:
 
 ```
 <MAJOR_VERSION> * 1,000,000 + <MINOR_VERSION> * 1,000 + <REVISION_VERSION>
@@ -125,29 +135,6 @@ which is compute by following formula:
 For example:
 - F-extension v2.2 will define `__riscv_f` as `2002000`.
 
-| Name                    | Value        | When defined                  |
-| ----------------------- | ------------ | ----------------------------- |
-| __riscv_arch_test       | 1            | Defined if compiler support new architecture extension test macro. |
-| __riscv_i               | Arch Version | `I` extension is available.   |
-| __riscv_e               | Arch Version | `E` extension is available.   |
-| __riscv_m               | Arch Version | `M` extension is available.   |
-| __riscv_a               | Arch Version | `A` extension is available.   |
-| __riscv_f               | Arch Version | `F` extension is available.   |
-| __riscv_d               | Arch Version | `D` extension is available.   |
-| __riscv_q               | Arch Version | `Q` extension is available.   |
-| __riscv_c               | Arch Version | `C` extension is available.   |
-| __riscv_p               | Arch Version | `P` extension is available.   |
-| __riscv_v               | Arch Version | `V` extension is available.   |
-| __riscv_zicsr           | Arch Version | `Zicsr` extension is available. |
-| __riscv_zifencei        | Arch Version | `Zifencei` extension is available. |
-| __riscv_zawrs           | Arch Version | `Zawrs` extension is available. |
-| __riscv_zba             | Arch Version | `Zba` extension is available. |
-| __riscv_zbb             | Arch Version | `Zbb` extension is available. |
-| __riscv_zbc             | Arch Version | `Zbc` extension is available. |
-| __riscv_zbs             | Arch Version | `Zbs` extension is available. |
-| __riscv_zfh             | Arch Version | `Zfh` extension is available. |
-| __riscv_zimop           | Arch Version | `Zimop` extension is available. |
-
 ### ABI Related Preprocessor Definitions
 
 | Name                     | Value | When defined                  |