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 binutils-2.20.1.orig/ld/ldmain.c binutils-2.20.1/ld/ldmain.c

--- binutils-2.20.1.orig/ld/ldmain.c	2010-08-17 19:32:17.000000000 +0200

+++ binutils-2.20.1/ld/ldmain.c	2010-08-17 19:32:18.000000000 +0200

@@ -264,6 +264,7 @@

   link_info.keep_memory = TRUE;

   link_info.new_dtags = TRUE;

   link_info.combreloc = TRUE;

+  link_info.relro = TRUE;

   link_info.strip_discarded = TRUE;

   link_info.emit_hash = TRUE;

 #ifndef __mips__