doc/go1.15: announce GO386=387 deprecation

For #40255.
Updates #37419.

Change-Id: If9210c855cc2eea079e7e469463d4203888748f7
Reviewed-on: https://go-review.googlesource.com/c/go/+/243137
Reviewed-by: Andrew Bonventre <[email protected]>

Author: aclements

doc/go1.15.html

@@ -94,6 +94,16 @@ <h3 id="riscv">RISC-V</h3>
   preemption.
 </p>
 
+<h3 id="386">386</h3>
+
+<p><!-- golang.org/issue/40255 -->
+  Go 1.15 is the last release to support x87-only floating-point
+  hardware (<code>GO386=387</code>). Future releases will require at
+  least SSE2 support on 386, raising Go's
+  minimum <code>GOARCH=386</code> requirement to the Intel Pentium 4
+  (released in 2000) or AMD Opteron/Athlon 64 (released in 2003).
+</p>
+
 <h2 id="tools">Tools</h2>
 
 <h3 id="go-command">Go command</h3>