Remove ARM assembly from RISC-V CYW43 SPI PIO

[earlephilhower]

Replace the ARM assembly for dual 16b swap with generic GCC builtin macros to allow building CYW43 SPI PIO driver for RISC-V

Fixes #2044

[kilograham]

This might be more compact

__force_inline static uint32_t __swap16x2(uint32_t a) {
#ifndef __riscv
    pico_default_asm ("rev16 %0, %0" : "+l" (a) : : );
#else
    uint32_t tmp;
    pico_default_asm (
        "rev8 %1, %0\n"
        "rori %0, %1, 16\n"
        : "+l" (a), "=l" (tmp));
#endif
    return a;
}

[earlephilhower]

Agreed. Quick check of generated assembly using __builtin gives

  5c:	6987d713          	rev8	a4,a5
  60:	83c1                	srli	a5,a5,0x10
  62:	8341                	srli	a4,a4,0x10
  64:	6987d793          	rev8	a5,a5
  68:	8fd9                	or	a5,a5,a4

22 bytes vs only 8 for the hand ASM.

  4a:	6987d713          	rev8	a4,a5
  4e:	61075793          	rori	a5,a4,0x10

I'll update and squash accordingly.

[earlephilhower]

Updated with hand-asm patch. Verified the assembly output does match the ARM and GCC builtins.