cmd/link: support trampoline insertion for PLT calls on ARM64
When internal linking with C objects, some C object relocations
may be turned into a CALL via PLT. For very large programs, the
PLT stub may be laid too far.
PLT stubs are generated late in the linker, and laid out after
the end of the text section. So if the text section is big, the
PLT stubs are likely too far.
To avoid this situation, add trampolines for PLT calls in the
trampoline pass. Only do this when the program is known too large
(i.e. the second pass of the two-pass algorithm).