Creating buildroot toolchain

[noamc]

I am trying to create riscv64 toolchain using buildroot.
I used sifive-next branch as a base to my work.
I used binutils-2.28, gcc-7, glibc-riscv-next, Linux-riscv-next (for Linux headers).

I managed to create working toolchain for riscv64 with this commit
However I found 3 issues I worked around and needed to get fixed.

1. GCC fails to build GLIBC without warning (I add to --disable-werror to configure)
   I think that GCC fails to see that size of -1L is same as a pointer.
   log is:
   ../sysdeps/unix/sysv/linux/mmap64.c: In function ‘__mmap64’:
   ../sysdeps/unix/sysv/linux/mmap64.c:45:12: error: cast to pointer from integer of different size [-Werror=int-to-pointer-cast]
   return (void *) INLINE_SYSCALL_ERROR_RETURN_VALUE (EINVAL);

2. Faild to build stage 1 compiler (aka inital gcc) due to misssing file
   log is:
   make[3]: *** No rule to make target '../../gcc/testsuite/selftests', needed by 's-selftest'. Stop.
   make[3]: *** Waiting for unfinished jobs....

3. missing kernel file needed by linux headers, add an empty one arch/riscv/include/uapi/asm/setup.h
   log is:
   scripts/Makefile.headersinst:72: *** Some mandatory headers (setup.h) are missing in arch/riscv/include/uapi/asm. Stop.
   scripts/Makefile.headersinst:26: recipe for target 'asm' failed

Note:
Still with this kernel spike is not booting, need to go back to master (4.6.y)