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/gcc/config/netbsd-elf.h gcc-4.4.0/gcc/config/netbsd-elf.h

--- gcc-4.4.0.orig/gcc/config/netbsd-elf.h	2007-09-03 18:14:04.000000000 +0200

+++ gcc-4.4.0/gcc/config/netbsd-elf.h	2009-05-27 21:38:14.000000000 +0200

@@ -82,6 +82,7 @@

 #define NETBSD_LINK_SPEC_ELF \

   "%{assert*} %{R*} %{rpath*} \

    %{shared:-shared} \

+   %{symbolic:-Bsymbolic} \

    %{!shared: \

      -dc -dp \

      %{!nostdlib: \