viml/profile: switch to uv_gettimeofday() #10356

Performance of high-resolution time (clock_gettime via uv_hrtime) is
expensive on some systems.  For profiling VimL, syntax, etc., we don't
care about nanosecond-precision and monotonicity edge-cases, so avoid
uv_hrtime().

closes #10328

From the uv__hrtime() source:
0cdb4a5b4b/src/unix/linux-core.c (L442-L462)

    /* Prefer CLOCK_MONOTONIC_COARSE if available but only when it has
     * millisecond granularity or better.  CLOCK_MONOTONIC_COARSE is
     * serviced entirely from the vDSO, whereas CLOCK_MONOTONIC may
     * decide to make a costly system call.
     */

This micro-benchmark (Debug build) shows negligible differences on my
system:

    #include <sys/time.h>
    ...

    proftime_T tm = profile_start();
    int trials = 999999;
    int64_t t = 0;
    struct timeval tv;
    for (int i = 0; i < trials; i++) {
      t += gettimeofday(&tv,NULL);
    }
    tm = profile_end(tm);
    ILOG("%d trials of gettimeofday: %s", trials, profile_msg(tm));
    tm = profile_start();
    for (int i = 0; i < trials; i++) {
      t += os_hrtime();
    }
    tm = profile_end(tm);
    ILOG("%d trials of os_hrtime: %s", trials, profile_msg(tm));
    tm = profile_start();
    for (int i = 0; i < trials; i++) {
      t += os_utime();
    }
    tm = profile_end(tm);
    ILOG("%d trials of os_utime: %s", trials, profile_msg(tm));
    ILOG("%zu", t);

src/nvim/eval.c

@@ -16536,7 +16536,7 @@ static void f_reltimefloat(typval_T *argvars , typval_T *rettv, FunPtr fptr)
   rettv->v_type = VAR_FLOAT;
   rettv->vval.v_float = 0;
   if (list2proftime(&argvars[0], &tm) == OK) {
-    rettv->vval.v_float = ((float_T)tm) / 1000000000;
+    rettv->vval.v_float = ((float_T)tm) / 1000000;
   }
 }