llext: Support non-paired RISC-V PCREL Relocation

Currently, RISC-V's architecture-specific relocations assume that
all relocations of type R_RISCV_PCREL_LO12_I and _S are processed
immediately after the R_RISCV_PCREL_HI20 relocation that they
share a relocation target with. While this is the case most of
the time, the RISC-V PSABI specification does not guarantee that.
This commit corrects this by determining the R_RISCV_PCREL_HI20
relocation based on the symbol value of the R_RISCV_PCREL_LO12
relocation, as specified in the PSABI.

Signed-off-by: Eric Ackermann <eric.ackermann@cispa.de>