gnu: gcc: Use `gcc-configure-flags-for-triplet' when cross-compiling GCC.

* gnu/packages/cross-base.scm (gcc-configure-flags-for-triplet): Move to...
* gnu/packages/gcc.scm (gcc-configure-flags-for-triplet): ... here.  New
  procedure.
  (gcc-4.7): Use it when (%current-target-system) is true.

gnu: cross-base: Add support for `mips64el-linux-gnu-gnuabi64' (N64 ABI).

* gnu/packages/cross-base.scm (gcc-configure-flags-for-triplet): New
  procedure.
  (cross-gcc): Use it.
  (xgcc-mips64el): Use *-gnuabi64 instead of the N32 ABI.

gnu: cross-base: Make `glibc-dynamic-linker' do the right thing.

* gnu/packages/bootstrap.scm (glibc-dynamic-linker): Determine SYSTEM
  based on (%current-target-system) when it's true.
* gnu/packages/cross-base.scm (cross-gcc): Directly call
  (package-arguments gcc-4.7) from the body of `arguments'.

gnu: cross-base: Build Binutils with `--with-sysroot=/'.

* gnu/packages/cross-base.scm (cross-binutils): Use `--with-sysroot=/'.

gnu: cross-base: Make the cross tools native inputs.

* gnu/packages/cross-base.scm (cross-gcc): Change `inputs' to
  `native-inputs', and set `inputs' to the empty list.
  (cross-libc): Likewise.

gnu: cross-binutils: Build with `--with-sysroot'.

This fixes resolution of DT_NEEDED entries in cross-built libraries.
For instance, if ltdl.so needs libdl.so and has it in its RUNPATH, then
libdl.so is searched for in the right place.

* gnu/packages/cross-base.scm (cross-binutils): Pass
  `--with-sysroot=/no-such-path'.

build-system/gnu: Implement cross build.

* guix/build-system/gnu.scm (inputs-search-paths): New procedure.
  (standard-search-paths): Use it.
  (expand-inputs): New procedure.
  (standard-inputs): Use it.
  (standard-cross-packages, standard-cross-inputs,
  standard-cross-search-paths, gnu-cross-build): New procedures.
  (gnu-build-system): Set `cross-build' field to `gnu-cross-build'.
* gnu/packages/cross-base.scm: Export `cross-gcc', `cross-binutils', and
  `cross-libc'.
* guix/build/gnu-cross-build.scm: New file.
* Makefile.am (MODULES): Add it.

gnu: cross-gcc: Set the right search paths.

* gnu/packages/cross-base.scm (cross-gcc): Override `search-paths' and
  `native-search-paths'.

Merge branch 'core-updates'

gnu: Add missing import in (gnu packages cross-base).

* gnu/packages/cross-base.scm: Use (gnu packages gcc).
  Reported by Nikita Karetnikov <nikita@karetnikov.org>.

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".