doc: update go1.5.txt (gc constant arithmetic now based on math/big)

Change-Id: Iff943d15e83e7db1f3c77f509a60e00ee2041d69
Reviewed-on: https://go-review.googlesource.com/8592
Reviewed-by: Robert Griesemer <[email protected]>

Author: griesemer

doc/go1.5.txt

@@ -49,6 +49,7 @@ text/template: add Options method (https://golang.org/cl/8462)
 Tools:
 
 build: external linking support for windows (https://golang.org/cl/7163, 7282, 7283, 7284, 7534, 7535)
+cmd/gc: constant arithmetic is based on math/big (https://golang.org/cl/7830, 7851, 7857, 8426, 7858, 7912, 8171)
 cmd/go, go/build: add ${SRCDIR} variable expansion to cgo lines (https://golang.org/cl/1756)
 cmd/go: add $DOLLAR to generate's variables (https://golang.org/cl/8091)
 cmd/go: std wildcard now excludes commands in main repo (https://golang.org/cl/5550)