riscv: optimize gcd() code size when CONFIG_RISCV_ISA_ZBB is disabled

The binary GCD implementation depends on efficient ffs(), which on RISC-V
requires hardware support for the Zbb extension. When
CONFIG_RISCV_ISA_ZBB is not enabled, the kernel will never use binary GCD,
as runtime logic will always fall back to the odd-even implementation.

To avoid compiling unused code and reduce code size, select
CONFIG_CPU_NO_EFFICIENT_FFS when CONFIG_RISCV_ISA_ZBB is not set.

$ ./scripts/bloat-o-meter ./lib/math/gcd.o.old ./lib/math/gcd.o.new
add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-274 (-274)
Function old new delta
gcd 360 86 -274
Total: Before=384, After=110, chg -71.35%

Link: https://lkml.kernel.org/r/20250606134758.1308400-3-visitorckw@gmail.com
Co-developed-by: Yu-Chun Lin <eleanor15x@gmail.com>
Signed-off-by: Yu-Chun Lin <eleanor15x@gmail.com>
Signed-off-by: Kuan-Wei Chiu <visitorckw@gmail.com>
Acked-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Cc: Albert Ou <aou@eecs.berkeley.edu>
Cc: Ching-Chun (Jim) Huang <jserv@ccns.ncku.edu.tw>
Cc: Palmer Dabbelt <palmer@dabbelt.com>
Cc: Paul Walmsley <paul.walmsley@sifive.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>