cmd/link/internal/amd64: always generate R_X86_64_PLT32 for SDYNIMPORT calls

Currently, in the non-DynlinkingGo case with external linking, we generate a
R_X86_64_GOTPCREL relocation for the imported symbol. This results in the
external linker turning this into a R_X86_64_GLOB_DAT relocation, rather
than a R_X86_64_JUMP_SLOT. Always generate R_X86_64_PLT32 for SDYNIMPORT
calls so that these calls work correctly.

Update #36435
Fixes #42671

Change-Id: I8a28884b7853cb4135053ed817bedc919482f4ad
Reviewed-on: https://go-review.googlesource.com/c/go/+/270377


Trust: Joel Sing <joel@sing.id.au>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>