ARM Fixes

Few changes for ARM ThunderX2 and ARM-clang:

• Do not assume char is signed. Use int8_t for the time-bin .
• Improve the detection of the ARMv8.1a cycle counter assembly instruction.
• Fix cycle.h for ARMv8.1a. Will push upstream to FFTW.
• Add a synchronization point in the memswap() for clang on ARM.
• Do not let the autotools choose the flags for clang and GCC on ARM 8.1a. Use the native instructions.
• Upgrade to some of the autotools macros.