cmd/go: remove GOAMD64 environment variable

This removes the GOAMD64 environment variable and its documentation.
The value is instead supplied by a compiled-in constant.

Note that function alignment is also dependent on the value of
the (removed) flag; it is 32 for aligned jumps, 16 if not.
When the flag-dependent logic is removed, it will be 32.

Updates #35881.

Change-Id: Ic41c0b9833d2e8a31fa3ce8067d92aa2f165bf72
Reviewed-on: https://go-review.googlesource.com/c/go/+/231600


Reviewed-by: Austin Clements <austin@google.com>