Error out early if the architecture is not supported

[lu-zero]

This would make users trying to build it on PowerPC or RISC-V less confused.

Example:

lu_zero@spacemit ~/rr $ cmake -B build && cmake --build build
CMake Warning (dev) at CMakeLists.txt:363 (message):
  Couldn't find rtld-audit support.  librraudit skipped.
This warning is for project developers.  Use -Wno-dev to suppress it.

CMake Warning (dev) at CMakeLists.txt:371 (message):
  backtrace(3) not present in execinfo.h.  Automatic backtraces for failures
  in rr are disabled.
This warning is for project developers.  Use -Wno-dev to suppress it.

-- Configuring done (2.7s)
-- Generating done (12.5s)
-- Build files have been written to: /home/lu_zero/rr/build
[  0%] Building C object CMakeFiles/brotli.dir/third-party/brotli/common/constants.c.o
cc: error: unrecognized command-line option ‘-msse2’
gmake[2]: *** [CMakeFiles/brotli.dir/build.make:76: CMakeFiles/brotli.dir/third-party/brotli/common/constants.c.o] Error 1
gmake[1]: *** [CMakeFiles/Makefile2:2722: CMakeFiles/brotli.dir/all] Error 2
gmake: *** [Makefile:166: all] Error 2
lu_zero@spacemit ~/rr $ uname -a
Linux spacemit 6.1.15 #1.0~rc1 SMP PREEMPT Mon Apr 29 09:05:59 UTC 2024 riscv64 GNU/Linux

[rocallahan]

Thanks!