RISC-V: Remove non-existent `Zb`
The `Zb` extension does not exist and we instead have the `B` extension
which is a superset of the three subextensions: `Zba`, `Zbb` and `Zbs`.
For now (since `B` extension is not stabilized on Rust),
this commit just removes `Zb`.
diff --git a/src/attributes/codegen.md b/src/attributes/codegen.md
index 8d79481..2abcd83 100644
--- a/src/attributes/codegen.md
+++ b/src/attributes/codegen.md
@@ -503,7 +503,6 @@
`a` | | [A][rv-a] --- Atomic instructions
`c` | | [C][rv-c] --- Compressed instructions
`m` | | [M][rv-m] --- Integer Multiplication and Division instructions
-`zb` | `zba`, `zbc`, `zbs` | [Zb][rv-zb] --- Bit Manipulation instructions
`zba` | | [Zba][rv-zb-zba] --- Address Generation instructions
`zbb` | | [Zbb][rv-zb-zbb] --- Basic bit-manipulation
`zbc` | | [Zbc][rv-zb-zbc] --- Carry-less multiplication
@@ -527,7 +526,6 @@
[rv-a]: https://github.com/riscv/riscv-isa-manual/blob/de46343a245c6ee1f7b1a40c92fe1a86bd4f4978/src/a-st-ext.adoc
[rv-c]: https://github.com/riscv/riscv-isa-manual/blob/de46343a245c6ee1f7b1a40c92fe1a86bd4f4978/src/c-st-ext.adoc
[rv-m]: https://github.com/riscv/riscv-isa-manual/blob/de46343a245c6ee1f7b1a40c92fe1a86bd4f4978/src/m-st-ext.adoc
-[rv-zb]: https://github.com/riscv/riscv-bitmanip
[rv-zb-zba]: https://github.com/riscv/riscv-bitmanip/blob/main/bitmanip/zba.adoc
[rv-zb-zbb]: https://github.com/riscv/riscv-bitmanip/blob/main/bitmanip/zbb.adoc
[rv-zb-zbc]: https://github.com/riscv/riscv-bitmanip/blob/main/bitmanip/zbc.adoc
