lock: Use stdatomic.h (#56) ...

Provides the same assembly output while using the standardized interface

e.g.

0000000000000000 <set_priv_smc_in_progress>:
   0:   90000000    adrp    x0, 0 <set_priv_smc_in_progress>
   4:   91000000    add x0, x0, #0x0
   8:   52800022    mov w2, #0x1                    // #1
   c:   d503201f    nop
  10:   085ffc01    ldaxrb  w1, [x0]
  14:   08037c02    stxrb   w3, w2, [x0]
  18:   35ffffc3    cbnz    w3, 10 <set_priv_smc_in_progress+0x10>
  1c:   72001c3f    tst w1, #0xff
  20:   54ffff81    b.ne    10 <set_priv_smc_in_progress+0x10>  // b.any
  24:   d65f03c0    ret

Move source files to src/, add Makefile, fix all build and linkage errors, etc.