gcc: add support for gcc 6

gcc: add support for gcc 6

This commit adds the support for gcc 6. This release allows to remove
a large number of our gcc patches, mainly thanks to the Xtensa and
musl related patches being merged upstream.

Patches kept with no changes:

 100-uclibc-conf.patch
 301-missing-execinfo_h.patch
 810-arm-softfloat-libgcc.patch
 830-arm_unbreak_armv4t.patch
 840-microblaze-enable-dwarf-eh-support.patch
 860-cilk-wchar.patch
 890-fix-m68k-compile.patch

Patches dropped because they have been merged upstream, or were
already upstream backports:

 120-gcc-config.gcc-fix-typo-for-powerpc-e6500-cpu_is_64b.patch (merged)
 850-libstdcxx-uclibc-c99.patch (merged in a different form, see
 https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58393)
 870-xtensa-add-mauto-litpools-option.patch (upstream backport)
 871-xtensa-reimplement-register-spilling.patch (upstream backport)
 872-xtensa-use-unwind-dw2-fde-dip-instead-of-unwind-dw2-.patch (upstream backport)
 873-xtensa-fix-_Unwind_GetCFA.patch (upstream backport)
 874-xtensa-add-uclinux-support.patch (upstream backport)
 900-libitm-fixes-for-musl-support.patch (upstream backport)
 901-fixincludes-update-for-musl-support.patch (upstream backport)
 902-unwind-fix-for-musl.patch (upstream backport)
 903-libstdc++-libgfortran-gthr-workaround-for-musl.patch (upstream backport)
 904-musl-libc-config.patch (upstream backport)
 905-add-musl-support-to-gcc.patch (upstream backport)
 905-add-musl-support-to-gcc.patch (upstream backport)
 906-mips-musl-support.patch (upstream backport)
 907-x86-musl-support.patch (upstream backport)
 908-arm-musl-support.patch (upstream backport)
 909-aarch64-musl-support.patch (upstream backport)

Successfully build-time and run-time tested with
qemu_arm_vexpress_defconfig, using gcc 6.x, both in uClibc and musl
configurations.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>