mingw32: enable sysrooted toolchains

When targeting mingw32, gcc expects to find its include files
in "mingw/include" instead of the traditional "usr/include".

[Yann E. MORIN: split the original patch]

--- binutils/ld/ldmain.c

+++ binutils/ld/ldmain.c

@@ -293,2 +293,3 @@ main (int argc, char **argv)

   link_info.combreloc = TRUE;

+  link_info.relro = TRUE;

   link_info.strip_discarded = TRUE;