Work around lld's inability to handle undefined weak symbols on risc-v.

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
kernel.

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 work around 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 that depend on PC-relative
addressing to load physical addresses before paging is enabled have 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.

Submitted by: jrtc27
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D23064