Add cpu / mem / fsz limits to build system

Thanks to all the refactorings we now have the ability to enforce
reasonable limitations on the amount of resources any individual
compile or test can consume. Those limits are currently:

- `-C 8` seconds of 3.1ghz CPU time
- `-M 256mebibytes` of virtual memory
- `-F 100megabyte` limit on file size

Only one file currently needs to exceed these limits:

    o/$(MODE)/third_party/python/Objects/unicodeobject.o: \
        QUOTA += -C16  # overrides cpu limit to 16 seconds

This change introduces a new sizetol() function to LIBC_FMT for parsing
byte or bit size strings with Si unit suffixes. Functions like atoi()
have been rewritten too.

libc/fmt/atoi.c

@@ -16,21 +16,34 @@
 │ TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR             │
 │ PERFORMANCE OF THIS SOFTWARE.                                                │
 ╚─────────────────────────────────────────────────────────────────────────────*/
+#include "libc/errno.h"
 #include "libc/fmt/conv.h"
 #include "libc/limits.h"
+#include "libc/str/str.h"
 
 /**
- * Decodes decimal number from ASCII string.
+ * Decodes decimal integer from ASCII string.
  *
- * @param s is a non-null NUL-terminated string
- * @return the decoded signed saturated number
- * @note calling strtoimax() directly with base 0 permits greater
- *     flexibility in terms of inputs
+ * @param s is a non-null nul-terminated string
+ * @return the decoded signed saturated integer
  */
 int atoi(const char *s) {
-  int res;
-  res = strtoimax(s, NULL, 10);
-  if (res < INT_MIN) return INT_MIN;
-  if (res > INT_MAX) return INT_MAX;
-  return res;
+  int x, c, d;
+  do {
+    c = *s++;
+  } while (c == ' ' || c == '\t');
+  d = c == '-' ? -1 : 1;
+  if (c == '-' || c == '+') c = *s++;
+  for (x = 0; isdigit(c); c = *s++) {
+    if (__builtin_mul_overflow(x, 10, &x) ||
+        __builtin_add_overflow(x, (c - '0') * d, &x)) {
+      errno = ERANGE;
+      if (d > 0) {
+        return INT_MAX;
+      } else {
+        return INT_MIN;
+      }
+    }
+  }
+  return x;
 }