[NFC] clang-format nativecpu_utils.cpp. (#17570)

We had turned off clang-formatting for nativecpu_utils.cpp because the
macro definitions we used confused the formatter to the point of making
the result unreadable, but by changing the macros so that they are
followed by a semicolon, and by avoiding NCPUPREFIX and spelling out the
concatenation directly, we can get it into a state where it does work
for clang-format.

Author: hvdijk