cmd/{compile, link}: support mapping symbols for ARM64

[vpachkov]

ELF for the Arm® 64-bit Architecture (AArch64):
Mapping symbols chapter
[https://github.com/ARM-software/abi-aa/blob/2020q4/aaelf64/aaelf64.rst#mapping-symbols]
requires that the special symbols are inserted into object files:
$x - At the start of a region of code containing AArch64 instructions.
$d - At the start of a region of data.

This patch introduces special mapping symbols that identify inline
transitions between code and literal pool boundaries.

Resolves #47908

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