cmd/link: ignore mapping symbols on riscv64

[ziyao233]

Specified in RISC-V ELF psABI1, mapping symbols are symbols starting
with "$d" or "$x" with STT_NOTYPE, STB_LOCAL and zero sizes, indicating
boundaries between code and data in the same section.

Let's simply ignore them as they're only markers instead of real symbols.
This fixes linking errors like

sym#63 ("$d"): ignoring symbol in section 4 (".riscv.attributes") (type 0)

when using CGO together with Clang and internal linker, which are caused
by unnecessary (but technically correct) mapping symbols created by LLVM
for various sections.

Fixes #73591

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