[RISCV] Assemble `call foo` to R_RISCV_CALL_PLT

R_RISCV_CALL/R_RISCV_CALL_PLT distinction isn't necessary. R_RISCV_CALL has been
deprecated as a resolution to
https://github.com/riscv-non-isa/riscv-elf-psabi-doc/issues/98 .

ld.lld and mold treat the two relocation types the same. GNU ld has a custom
handling for undefined weak functions which is unnecessary: calling an
unresolved undefined weak function is UB and GNU ld can handle the case without
a relocation error (such a function call is usually guarded by a zero value
check and should be allowed).

This patch assembles `call foo` to use R_RISCV_CALL_PLT instead of the
deprecated R_RISCV_CALL.

Note: the code generator still differentiates `call foo` and (maybe preemptible)
`call foo@plt`, but the difference is purely aesthetic.

Note: D105429 does not support R_RISCV_CALL_PLT correctly. Changed the test to
force R_RISCV_CALL for now.

Reviewed By: kito-cheng

Differential Revision: https://reviews.llvm.org/D132530