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)

 # MIPS specific config options

 config ARCH_mips

     select ARCH_SUPPORTS_32

     select ARCH_SUPPORTS_64 if EXPERIMENTAL

     select ARCH_DEFAULT_32

     select ARCH_USE_MMU

     select ARCH_SUPPORTS_BOTH_ENDIAN

     select ARCH_DEFAULT_BE

     select ARCH_SUPPORT_ARCH

     select ARCH_SUPPORT_TUNE

     help

       The MIPS architecture, as defined by:

         http://www.mips.com/