[openmp] 1.5x inference speedup

Problem:
- clock is CPU and doesn't work properly with parallel execution.
- perf execution is matmul x weights bound.

Solution:
- use gettimeofday instead.
- utilize openmp to parallelize matmul.

Note:
- if not compiled with -fopenmp the #pragma is ignored and single
  execution is performed.
- there are additional env variable to setup for openmp (optinally)
  to setup the number of threads, scheduler etc.

Benchmarks:
```
clang -Ofast -march=native  run.c  -lm  -o run          // achieved tok/s: 340.878828
clang -Ofast -fopenmp -march=native  run.c  -lm  -o run // achieved tok/s: 524.590164
```

run.c

@@ -13,6 +13,7 @@ $ ./run
 #include <time.h>
 #include <math.h>
 #include <string.h>
+#include <sys/time.h>
 
 // ----------------------------------------------------------------------------
 // Transformer and RunState structs, and related memory management
@@ -212,6 +213,7 @@ void softmax(float* x, int size) {
 
 void matmul(float* xout, float* x, float* w, int n, int d) {
     // W (d,n) @ x (n,) -> xout (d,)
+    #pragma omp parallel for
     for (int i = 0; i < d; i++) {
         float val = 0.0f;
         for (int j = 0; j < n; j++) {
@@ -372,6 +374,12 @@ int argmax(float* v, int n) {
 
 // ----------------------------------------------------------------------------
 
+long time_in_ms() {
+  struct timeval time;
+  gettimeofday(&time, NULL);
+  return time.tv_sec * 1000 + time.tv_usec / 1000;
+}
+
 int main(int argc, char *argv[]) {
 
     // poor man's C argparse
@@ -438,7 +446,8 @@ int main(int argc, char *argv[]) {
     malloc_run_state(&state, &config);
     
     // the current position we are in
-    clock_t start = clock();
+    long start = time_in_ms();
+
     int next;
     int token = 1; // 1 = BOS token in Llama-2 sentencepiece
     int pos = 0;
@@ -469,9 +478,8 @@ int main(int argc, char *argv[]) {
     printf("\n");
 
     // report our achieved tok/s
-    clock_t end = clock();
-    double elapsed = (double)(end - start) / CLOCKS_PER_SEC;
-    printf("achieved tok/s: %f\n", config.seq_len / elapsed);
+    long end = time_in_ms();
+    printf("achieved tok/s: %f\n", config.seq_len / (double)(end-start)*1000);
 
     // memory cleanup
     free_run_state(&state);