tests/bitops: starting point for rtapi_bitops.h tests

more creativity needed
configure: add USE_GCC_ATOMIC_OPS, --disable-gcc-atomic-ops
rtapi_bitops.h: proposed replacement by gcc intrinsics

the code for test_bit() might need improvement; however since most (if not
all) use of test_bit() in LinuxCNC will use __constant_test_bit() I dont
see much downside.
config.h/BUFFERLEN: remove redefinitions

including config.h defines BUFFERLEN which causes some fallout
rtapi_bitops.h: remove blunder, use newer intrinsics
rtapi_bitops.h: fix conditionals
rtapi_bitops.h: use builtin operations
rtapi.h: amend for new bitops syntax
configure: remove test for bultin atomic operations
rtapi_bitops: adapt usage to new API
rtapi_bitops.h: certain macros will become available with C11 only
configure.in: remove leftover subst of USE_GCC_ATOMIC_OPS
rtapi_bitops.h: assume a byte has 8 bits