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

win32/tcc-win32.txt

@@ -18,6 +18,12 @@
     system PATH.
 
 
+    Include and library search paths
+    --------------------------------
+    On windows,  the standard "include" and "lib" directories are searched
+    relatively from the location of the executables (tcc.exe, libtcc.dll).
+
+
     Examples:
     ---------
     Open a console window (DOS box) and 'cd' to the examples directory.