libc/mingw: do not remove support symlink

Under mingw, it seems that there is a mix between the traditional /usr
directory, and a similar-purposed /mingw directory (both in the sysroot).

Currently, we create /mingw as a symlink to /usr, and we removed it in
the libc-finish step.

Unfortunately, this prevents the pre-processor to find the headers.
Keeping the symlink makes it magically work...

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>
(transplanted from fa5c70b11fecf390c78780fe8f8ba0a836a59e92)

diff -durN gcc-4.4.0.orig/libgomp/configure gcc-4.4.0/libgomp/configure

--- gcc-4.4.0.orig/libgomp/configure	2009-04-21 11:08:08.000000000 +0200

+++ gcc-4.4.0/libgomp/configure	2009-05-27 21:38:19.000000000 +0200

@@ -3334,7 +3334,7 @@

 # Add -Wall -Werror if we are using GCC.

 if test "x$GCC" = "xyes"; then

-  XCFLAGS="$XCFLAGS -Wall -Werror"

+  XCFLAGS="$XCFLAGS -Wall"

 fi

 # Find other programs we need.