cmd/5a, cmd/5c, cmd/6a, cmd/6c, cmd/8a, cmd/8c, cmd/ld: update reference

Reference the 80386 compiler documentation now that the
documentation for the 68020 is offline.

R=golang-dev, minux.ma, rsc
CC=golang-dev
https://golang.org/cl/7127053

src/cmd/5a/doc.go

@@ -6,7 +6,7 @@
 
 5a is a version of the Plan 9 assembler.  The original is documented at
 
-	http://plan9.bell-labs.com/magic/man2html/1/2a
+	http://plan9.bell-labs.com/magic/man2html/1/8a
 
 Its target architecture is the ARM, referred to by these tools as arm.
 

src/cmd/5c/doc.go

@@ -6,7 +6,7 @@
 
 5c is a version of the Plan 9 C compiler.  The original is documented at
 
-	http://plan9.bell-labs.com/magic/man2html/1/2c
+	http://plan9.bell-labs.com/magic/man2html/1/8c
 
 Its target architecture is the ARM, referred to by these tools as arm.
 

src/cmd/6a/doc.go

@@ -6,7 +6,7 @@
 
 6a is a version of the Plan 9 assembler.  The original is documented at
 
-	http://plan9.bell-labs.com/magic/man2html/1/2a
+	http://plan9.bell-labs.com/magic/man2html/1/8a
 
 Its target architecture is the x86-64, referred to by these tools as amd64.
 

src/cmd/6c/doc.go

@@ -6,7 +6,7 @@
 
 6c is a version of the Plan 9 C compiler.  The original is documented at
 
-	http://plan9.bell-labs.com/magic/man2html/1/2c
+	http://plan9.bell-labs.com/magic/man2html/1/8c
 
 Its target architecture is the x86-64, referred to by these tools as amd64.
 

src/cmd/8a/doc.go

@@ -6,7 +6,7 @@
 
 8a is a version of the Plan 9 assembler.  The original is documented at
 
-	http://plan9.bell-labs.com/magic/man2html/1/2a
+	http://plan9.bell-labs.com/magic/man2html/1/8a
 
 Its target architecture is the x86, referred to by these tools for historical reasons as 386.
 

src/cmd/8c/doc.go

@@ -6,7 +6,7 @@
 
 8c is a version of the Plan 9 C compiler.  The original is documented at
 
-	http://plan9.bell-labs.com/magic/man2html/1/2c
+	http://plan9.bell-labs.com/magic/man2html/1/8c
 
 Its target architecture is the x86, referred to by these tools for historical reasons as 386.
 

src/cmd/ld/doc.go

@@ -6,7 +6,7 @@
 
 Ld is the portable code for a modified version of the Plan 9 linker.  The original is documented at
 
-	http://plan9.bell-labs.com/magic/man2html/1/2l
+	http://plan9.bell-labs.com/magic/man2html/1/8l
 
 It reads object files (.5, .6, or .8 files) and writes a binary named for the
 architecture (5.out, 6.out, 8.out) by default (if $GOOS is windows, a .exe suffix