atomic: fix compilation for C++ and Windows-ARM64

[alfredh]

original error for C++ file on Windows-ARM64:

C:\PROGRA~1\MICROS~2\2022\ENTERP~1\VC\Tools\MSVC\1442~1.344\bin\Hostx64\arm64\cl.exe  /nologo /TP -DARCH=\"AMD64\" -DHAVE_ATOMIC -DHAVE_IO_H -DHAVE_PQOS_FLOWID -DHAVE_QOS_FLOWID -DHAVE_SELECT -DHAVE_UNIXSOCK=1 -DOS=\"Windows\" -DRE_VERSION=\"3.18.0\" -DVER_MAJOR=3 -DVER_MINOR=18 -DVER_PATCH=0 -DWIN32 -D_CRT_SECURE_NO_WARNINGS -D_GNU_SOURCE -D_WIN32_WINNT=0x0600 -Dretest_EXPORTS -ID:\a\re\re\test\. -ID:\a\re\re\include /DWIN32 /D_WINDOWS /EHsc /Ob0 /Od /RTC1 -MDd -Zi /W3 /showIncludes /Fotest\CMakeFiles\retest.dir\cplusplus.cpp.obj /Fdtest\CMakeFiles\retest.dir\ /FS -c D:\a\re\re\test\cplusplus.cpp D:\a\re\re\include\re_atomic.h(602): error C2664: 'char __iso_volatile_load8(volatile const char *)': cannot convert argument 1 from 'const unsigned char *' to 'volatile const char *' D:\a\re\re\include\re_atomic.h(602): note: Types pointed to are unrelated; conversion requires reinterpret_cast, C-style cast or parenthesized function-style cast

I am not an expert on atomics, perhaps there are better solutions.