i386-gen: preserve fp control word in gen_cvt_ftoi

- Use runtime function for conversion
- Also initialize fp with tcc -run on windows

This fixes a bug where
  double x = 1.0;
  double y = 1.0000000000000001;
  double z = x < y ? 0 : sqrt (x*x - y*y);
caused a bad sqrt because rounding precision for the x < y comparison
was different to the one used within the sqrt function.

This also fixes a bug where
  printf("%d, %d", (int)pow(10, 2), (int)pow(10, 2));
would print
  100, 99

Unrelated:
  win32: document relative include & lib lookup
  win32: normalize_slashes: do not mirror silly gcc behavior
  This reverts part of commit 8a81f9e103
  winapi: add missing WINAPI decl. for some functions

tccrun.c

@@ -97,7 +97,7 @@ LIBTCCAPI int tcc_run(TCCState *s1, int argc, char **argv)
     if (tcc_relocate(s1, TCC_RELOCATE_AUTO) < 0)
         return -1;
 
-    prog_main = tcc_get_symbol_err(s1, "main");
+    prog_main = tcc_get_symbol_err(s1, s1->runtime_main);
 
 #ifdef CONFIG_TCC_BACKTRACE
     if (s1->do_debug) {