This:

echo 'int main (void) { return 0; }' > t.c
gcc `rpm --eval "%build_cflags %build_ldflags"` -Wl,-z,execstack t.c
readelf -lW a.out | grep GNU_STACK
annocheck a.out

produces:

  GNU_STACK      0x000000 0x0000000000000000 0x0000000000000000 0x000000 0x000000 RWE 0x10

but no executable stack error from annocheck.

I would have expected an error from annocheck here.

Note that the PT_GNU_STACK program header is obviously not present in ET_REL files.  There, a special note section is used, .note.GNU-stack.  If the section is executable, the stack will be marked executable in the program header.  If the section is absent, the behavior is target-dependent.