gnu: cross-gcc: Change patch so that `CROSS_LIBRARY_PATH' is honored.

* gnu/packages/patches/gcc-cross-environment-variables.patch: Add two
  hunks, most notably one that changes GCC to honor LIBRARY_PATH_ENV
  when cross-compiling.

Merge branch 'core-updates'

gnu: Add cross tool chain.

* gnu/packages/cross-base.scm: New file.
* gnu/packages/patches/gcc-cross-environment-variables.patch: New file.
* Makefile.am (MODULES): Add cross-base.scm.
  (dist_patch_DATA): Add gcc-cross-environment-variables.patch.
* gnu/packages/base.scm (gcc-4.7): Use `LDFLAGS_FOR_TARGET' instead of
  `LDFLAGS_FOR_BUILD', and use `-B' instead of `-L'.
* gnu/packages/bootstrap.scm (glibc-dynamic-linker): Add case for
  "mips64el-linux".