Fix fio_atomic_xchange fallback implementation for older GCC

[Low-power]

An or operation can't exchange those 2 values as expected in fio_atomic_xchange, makes fio_unlock no effect.

[codecov-io]

Codecov Report

Merging #55 into master will increase coverage by 0.06%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #55      +/-   ##
==========================================
+ Coverage   50.26%   50.32%   +0.06%     
==========================================
  Files          33       33              
  Lines       13432    13432              
==========================================
+ Hits         6751     6760       +9     
+ Misses       6681     6672       -9

Impacted Files	Coverage Δ
lib/facil/fio.h	82.7% <ø> (ø)	⬆️
lib/facil/fio.c	58.82% <0%> (+0.19%)	⬆️

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[Low-power]

Although nand operation can't really exchange any values, but 0 and 1, this is enough to fix the fio_unlock bug, since it only use 0 and 1.

[Low-power]

I force pushed a new commit that uses __sync_val_compare_and_swap instead.

Reference: https://gcc.gnu.org/onlinedocs/gcc-4.1.0/gcc/Atomic-Builtins.html

[Low-power]

Sorry, the function names was missing a _ character in my last commit. Fixed now.

[boazsegev]

Thank you for the PR!
☺️👍🏻🎉👏🏻

[boazsegev]

I should probably find a way to test on a wider range of compilers, as well as testing with and without the makefile (since this is a possible use-case).

Thank you very much for exposing this 🙏🏻☺️