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/libiberty/Makefile.in gcc-4.4.0/libiberty/Makefile.in

--- gcc-4.4.0.orig/libiberty/Makefile.in	2008-10-22 15:30:19.000000000 +0200

+++ gcc-4.4.0/libiberty/Makefile.in	2009-05-27 21:38:29.000000000 +0200

@@ -227,6 +227,7 @@

 	  $(AR) $(AR_FLAGS) $(TARGETLIB) \

 	    $(REQUIRED_OFILES) $(EXTRA_OFILES) $(LIBOBJS); \

 	  $(RANLIB) $(TARGETLIB); \

+	  cp $(TARGETLIB) ../ ; \

 	  cd ..; \

 	else true; fi