os: fix swap_bytes_u64

[tankf33der]

Found via s390x port.

[huly-for-github]

Connected to Huly®: V_0.6-22426

[spytheman]

Please add a test case too in file_le_be_test.v, to prevent future regressions.

[spytheman]

I should have probably used the same approach as in reverse_bytes_u64 in vlib/net/conv/conv.v 🤔 , using vfmt off/vfmt on to keep the stair aligned (and easy to inspect/check):

    // vfmt off
    return ((a >> 56) & 0x00000000_000000FF) |
           ((a >> 40) & 0x00000000_0000FF00) |
           ((a >> 24) & 0x00000000_00FF0000) |
           ((a >> 8)  & 0x00000000_FF000000) |
           ((a << 8)  & 0x000000FF_00000000) |
           ((a << 24) & 0x0000FF00_00000000) |
           ((a << 40) & 0x00FF0000_00000000) |
           ((a << 56) & 0xFF000000_00000000)
    // vfmt on

[spytheman]

Excellent work. Thank you 🙇🏻‍♂️.