Move names of committers from source files to AUTHORS list
* src/atomic_ops/sysdeps/gcc/hppa.h (header comment): Move
"modified-by" information to AUTHORS file.
* src/atomic_ops/sysdeps/gcc/m68k.h (header comment): Likewise.
* src/atomic_ops/sysdeps/gcc/powerpc.h (header comment): Likewise.
* src/atomic_ops_malloc.c (header comment): Likewise.
* src/atomic_ops_stack.c (header comment): Likewise.
* tests/test_atomic.c (header comment): Likewise.
* tests/test_malloc.c (header comment): Likewise.
* tests/test_stack.c (header comment): Likewise.
* AUTHORS: Add more entries.
* src/atomic_ops/sysdeps/standard_ao_double_t.h: Add copyright header.