Add an env var to turn off use of SIMD instructions

docs/glossary.rst

@@ -230,3 +230,8 @@ Variables that kitty sets when running child programs
 
    Set to ``1`` when kitty is running a shell because of the ``--hold`` flag. Can
    be used to specialize shell behavior in the shell rc files as desired.
+
+.. envvar:: KITTY_NO_SIMD
+
+   Set it to any value to prevent kitty from using SIMD CPU vector
+   instructions.

kitty/simd-string.c

@@ -143,6 +143,9 @@ init_simd(void *x) {
     do_check();
 #endif
 #endif
+    if (getenv("KITTY_NO_SIMD")) {
+        has_avx2 = false; has_sse4_2 = false;
+    }
 
 #undef do_check
     if (has_avx2) {