Fix compare_double_and_swap_double for clang/x86 in PIC mode

Fix compare_double_and_swap_double for clang/x86 in PIC mode

* src/atomic_ops/sysdeps/gcc/x86.h
(AO_compare_double_and_swap_double_full): Re-implement for PIC mode
saving EBX to a local variable (instead of pushing it to stack) and
saving memory operand address to a register (edi which is manually
preserved), so that the whole code could also work even if EBX points
to memory operand (e.g., in Clang); test __PIC__ using ifdef (instead
of "if"); update comments; reformat code.