poly1305: rename files to sum_𝑥.s

Since the wrapper files are called sum_𝑥.go, it makes sense that the
assembly files would be named similarly.

Change-Id: I5c515008b86c7fedd04b940d7846b84dfccdba33
Reviewed-on: https://go-review.googlesource.com/30727
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

poly1305/sum_amd64.go

@@ -6,7 +6,7 @@
 
 package poly1305
 
-// This function is implemented in poly1305_amd64.s
+// This function is implemented in sum_amd64.s
 //go:noescape
 func poly1305(out *[16]byte, m *byte, mlen uint64, key *[32]byte)
 

poly1305/sum_arm.go

@@ -6,7 +6,7 @@
 
 package poly1305
 
-// This function is implemented in poly1305_arm.s
+// This function is implemented in sum_arm.s
 //go:noescape
 func poly1305_auth_armv6(out *[16]byte, m *byte, mlen uint32, key *[32]byte)