…em with duff routines

This patch provides a fix for a problem linking large arm32 binaries
with external linking, specifically R_CALLARM relocations against
runtime.duff* routines being flagged by the external linker as not
reaching.

What appears to be happening in the bug in question is that the Go
linker and the external linker are using slightly different recipes to
decide whether a given R_CALLARM relocation will "fit" (e.g. will not
require a trampoline). The Go linker is taking into account the addend
on the call reloc (which for calls to runtime.duffcopy or
runtime.duffzero is nonzero), whereas the external linker appears to
be ignoring the addend.

Example to illustrate:

   Addr      Size   Func
   -----     -----  -----
   ...
   XYZ       1024   runtime.duffcopy
   ...
   ABC       ...    mypackge.MyFunc
     + R0: R_CALLARM  o=8 a=848 tgt=runtime.duffcopy<0>

Let's say that the distance between ABC (start address of
runtime.duffcopy) and XYZ (start of MyFunc) is just over the
architected 24-bit maximum displacement for an R_CALLARM (let's say
that ABC-XYZ is just over the architected limit by some small value,
say 36). Because we're calling into runtime.duffcopy at offset 848,
however, the relocation does in fact fit, but if the external linker
isn't taking into account the addend (assuming that all calls target
the first instruction of the called routine), then we'll get a
"doesn't fit" error from the linker.

To work around this problem, revise the ARM trampoline generation code
in the Go linker that computes the trampoline threshold to ignore the
addend on R_CALLARM relocations, so as to harmonize the two linkers.

Fixes #58502.
Updates #58428.
Updates #58425.

Change-Id: I56e580c05b7b47bbe8edf5532a1770bbd700fbe5
Reviewed-on: https://go-review.googlesource.com/c/go/+/469275
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Than McIntosh <thanm@google.com>
(cherry picked from commit 0b5affb)
Reviewed-on: https://go-review.googlesource.com/c/go/+/471598