chore(lua): turn off all optimizations in debug mode

I'm on macOS right now, so not sure how this will look in GCC, but at
least with clang/lldb we find the smoking gun pretty directly here.

First, without `-O0`

    lldb --file nvim
    (lldb) r
    Process 33400 stopped
    * thread #3, stop reason = EXC_BAD_ACCESS (code=1, address=0x7669746967750013)
        frame #0: 0x0000000103411c08 commandt.so`commandt_calculate_match + 52
    commandt.so`commandt_calculate_match:
    ->  0x103411c08 <+52>: ldr    x8, [x8, #0x8]
        0x103411c0c <+56>: adds   x8, x8, #0x1
        0x103411c10 <+60>: cset   w8, eq
        0x103411c14 <+64>: mov    w9, #0x1
    Target 0: (nvim) stopped.
    (lldb) bt
    * thread #3, stop reason = EXC_BAD_ACCESS (code=1, address=0x29)
      * frame #0: 0x0000000103411c08 commandt.so`commandt_calculate_match + 52
        frame #1: 0x0000000103413174 commandt.so`match_thread + 252
        frame #2: 0x00000001be24826c libsystem_pthread.dylib`_pthread_start + 148

Same with `-O0`:

    Process 33773 stopped
    * thread #3, stop reason = EXC_BAD_ACCESS (code=1, address=0x7669746967750093)
        frame #0: 0x0000000103411c08 commandt.so`commandt_calculate_match(haystack=0x766974696775008b, matcher=0x0000600002166800) at match.c:132:38
       129       matchinfo_t m;
       130       /* long i; */
       131       float score = 1.0;
    -> 132       int compute_bitmasks = haystack->bitmask == UNSET_BITMASK;
       133       m.haystack = haystack;
       134       m.needle_p = matcher->needle;
       135       m.needle_length = matcher->needle_length;
    Target 0: (nvim) stopped.

lua/wincent/commandt/Makefile

@@ -6,7 +6,7 @@ CC = gcc
 CCFLAGS += -Wall -Wextra -Wno-unused-parameter
 
 ifdef DEBUG
-    CCFLAGS += -DDEBUG -g
+    CCFLAGS += -DDEBUG -g -O0
 endif
 
 DLLEXT := so