RISC-V: add zbb support to string functions

Add handling for ZBB extension and add support for using it as a
variant for optimized string functions.

Support for the Zbb-str-variants is limited to the GNU-assembler
for now, as LLVM has not yet acquired the functionality to
selectively change the arch option in assembler code.
This is still under review at
    https://reviews.llvm.org/D123515

Co-developed-by: Christoph Muellner <[email protected]>
Signed-off-by: Christoph Muellner <[email protected]>
Signed-off-by: Heiko Stuebner <[email protected]>
Reviewed-by: Conor Dooley <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Palmer Dabbelt <[email protected]>

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