lld on RISC-V is not yet able to handle undefined weak symbols for
non-PIC code in the code model (medany/medium) used by the RISC-V
Both GCC and clang emit an auipc / addi pair of instructions to
generate an address relative to the current PC with a 31-bit offset.
Undefined weak symbols need to have an address of 0, but the kernel
runs with PC values much greater than 2^31, so there is no way to
construct a NULL pointer as a PC-relative value. The bfd linker
rewrites the instruction pair to use lui / addi with values of 0
to force a NULL pointer address. (There are similar cases for
'ld' becoming auipc / ld that bfd rewrites to lui / ld with an
address of 0.)
To workaround this, compile the kernel with -fPIE when using lld.
This does not make the kernel position-independent, but it does
force the compiler to indirect address lookups through GOT entries
(so auipc / ld against a GOT entry to fetch the address). This
adds extra memory indirections for global symbols, so should be
disabled once lld is finally fixed.
A few 'la' instructions in locore really need to use auipc / addi
and not indirect via GOT entries, so change those to use 'lla'
which always uses auipc / addi for both PIC and non-PIC.