riscv: disable stack-protector for vDSO

Currently, building the vDSO with clang leads assembler errors like the
following:

  /tmp/vgettimeofday-1ae0d2.s: Assembler messages:
  /tmp/vgettimeofday-1ae0d2.s:28: Error: bad expression
  /tmp/vgettimeofday-1ae0d2.s:28: Error: illegal operands `auipc a2,%got_pcrel_hi(__stack_chk_guard)'

Disable the stack-protector for vDSO to fix these.

Link: https://github.com/ClangBuiltLinux/linux/issues/1112


Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>