typos, and clarification that Zcmp/Zcmb are for embedded CPUs

[tariqkurd-repo]

fixes for:

https://github.com/riscv/riscv-code-size-reduction/issues/190 https://github.com/riscv/riscv-code-size-reduction/pull/202 https://github.com/riscv/riscv-code-size-reduction/pull/203

[brucehoult]

lgtm