[RISCV] Vendor Relocations for Xqci extension (#135400)
This patch implements vendor relocation support for RISC-V, starting
with the Xqci extensions.
Vendor Relocations need to emit the vendor symbol, and add an
`R_RISCV_VENDOR` relocation against that symbol before the specific
vendor relocation itself.
This patch adds a `maybeAddVendorReloc` function which is called from
`addReloc`, to implement this functionality. Vendor identifier symbols
are cached, so that at most one is emitted per vendor identifier.
Vendor identifiers symbols do not interfere with identically named
symbols used by assembly.
Co-authored-by: Sam Elliott <quic_aelliott@quicinc.com>
