Add an assembly implementation of strchr() for RISC-V.
By eliminating stack frame management (prologue/epilogue) and optimizing
the function entries, the assembly version provides significant relative
gains for short strings where the fixed overhead of the C function is
most prominent. As string length increases, performance converges with
the generic C implementation.
Benchmark results (QEMU TCG, rv64):
Length | Original (MB/s) | Optimized (MB/s) | Improvement
-------|-----------------|------------------|------------
1 B | 21 | 22 | +4.8%
7 B | 113 | 121 | +7.1%
16 B | 195 | 202 | +3.6%
512 B | 376 | 389 | +3.5%
4096 B | 394 | 393 | -0.3%
Signed-off-by: Feng Jiang <jiangfeng@kylinos.cn>
Tested-by: Joel Stanley <joel@jms.id.au>
Link: https://patch.msgid.link/20260130025018.172925-8-jiangfeng@kylinos.cn
Signed-off-by: Paul Walmsley <pjw@kernel.org>
