Tests use GNU C extensions and fail to compile

[thomwiggers]

gcc -O3      -std=c11 -fsanitize=undefined,address -march=native -D _AMD64_ -D __NIX__ -D _FAST_ -D _MULX_ -D _ADX_ -L./lib434 tests/arith_tests-p434.c tests/test_extras.c -lsidh -lm -o arith_tests-p434
tests/test_extras.c: In function ‘cpucycles’:
tests/test_extras.c:37:5: error: ‘asm’ undeclared (first use in this function)
   37 |     asm volatile ("rdtsc\n\t" : "=a" (lo), "=d"(hi));
      |     ^~~
tests/test_extras.c:37:5: note: each undeclared identifier is reported only once for each function it appears in
tests/test_extras.c:37:8: error: expected ‘;’ before ‘volatile’
   37 |     asm volatile ("rdtsc\n\t" : "=a" (lo), "=d"(hi));
      |        ^~~~~~~~~
      |        ;

If you use -std=gnu11, these errors go away.