cmd/link/internal/ld: introduce -funcalign=N option

cmd/link/internal/ld: introduce -funcalign=N option

This patch adds linker option -funcalign=N that allows to set alignment
for function entries.

This CL is based on vasiliy.leonenko@gmail.com's cl/615736.

For #72130

Change-Id: I57e5c9c4c71a989533643fda63a9a79c5c897dea
Reviewed-on: https://go-review.googlesource.com/c/go/+/660996
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>