Similar to #132569 for RISC-V, replace the unofficial `@plt` and `@gotpcrel` relocation specifiers, currently only used by clang -fexperimental-relative-c++-abi-vtables, with %pltpcrel %gotpcrel. The syntax is not used in humand-written assembly code, and is not supported by GNU assembler. Also replace the recent `@funcinit` with `%funcinit(x)`.