Disable -fstack-clash-protection on riscv64:
not supported even by GCC 7.3.1 on this architecture.
Apparently it requires architecture-specific support. In any case it
does not work with GCC 7.3.1 on riscv64:
stage3:/# gcc --version
gcc (GCC) 7.3.1 20180129
Copyright (C) 2017 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
stage3:/# gcc -fstack-clash-protection
gcc: error: unrecognized command line option '-fstack-clash-protection'; did you mean '-fstack-protector'?
gcc: fatal error: no input files