doc/go1.5.txt: note new options for go, gc, asm, ld

Change-Id: I353ff7eb35b066a1a2693c087c9876adac8e3fd0
Reviewed-on: https://go-review.googlesource.com/8763Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/go1.5.txt

@@ -57,6 +57,12 @@ cmd/go: std wildcard now excludes commands in main repo (https://golang.org/cl/5
 cmd/go: .swig/.swigcxx files now require SWIG 3.0.6 or later
 cmd/vet: better validation of struct tags (https://golang.org/cl/2685)
 cmd/ld: no longer record build timestamp in Windows PE file header (https://golang.org/cl/3740)
+cmd/go: add -toolexec build option
+cmd/go: drop -ccflags build option
+cmd/go: add -asmflags build option
+cmd/go: add -buildmode build option
+cmd/gc: add -dynlink option (for amd64 only)
+cmd/ld: add -buildmode option
 cmd/trace: new command to view traces (https://golang.org/cl/3601)
 
 Performance:
@@ -82,6 +88,10 @@ regexp: port RE2's bitstate backtracker to the regexp package (https://golang.or
 
 Assembler:
 
+New cmd/asm tool (now use go tool asm, not go tool 6a)
+
+Assembler now supports -dynlink option.
+
 ARM assembly syntax has had some features removed.
 
 	- mentioning SP or PC as a hardware register