cmd/compile: remove encoding of safemode bit from export data

Removes the encoding of this bit which was ignored but left behind
for 1.7 to minimize pre-1.7 export format changes. See the issue
for more details.

Fixes #15772.

Change-Id: I46cd7a66ad4c6003b78c64295cf3bda503ebf2dd
Reviewed-on: https://go-review.googlesource.com/27201
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/gc/bexport.go

@@ -346,7 +346,6 @@ func export(out *bufio.Writer, trace bool) int {
 	}
 
 	// write compiler-specific flags
-	p.bool(safemode)
 	if p.trace {
 		p.tracef("\n")
 	}

src/cmd/compile/internal/gc/bimport.go

@@ -106,9 +106,6 @@ func Import(in *bufio.Reader) {
 
 	// read compiler-specific flags
 
-	// read but ignore safemode bit (see issue #15772)
-	p.bool() // formerly: importpkg.Safe = p.bool()
-
 	// phase 2
 	objcount = 0
 	for {

src/cmd/compile/internal/gc/builtin.go

@@ -103,9 +103,9 @@ const runtimeimport = "" +
 	"write\x00\x01\x16d\x00\t\x19racereadrange\x00\x04\x16\raddr·1\x00d\x16\r" +
 	"size·2\x00d\x00\t\x1bracewriterange\x00\x04\x16\x98\x03\x00d\x16\x9a\x03\x00d\x00\t" +
 	"\x0fmsanread\x00\x04\x16\x98\x03\x00d\x16\x9a\x03\x00d\x00\t\x11msanwrite\x00\x04\x16\x98\x03\x00d" +
-	"\x16\x9a\x03\x00d\x00\v\xf8\x01\x02\v\x00\x01\x00\n$$\n"
+	"\x16\x9a\x03\x00d\x00\v\xf8\x01\v\x00\x01\x00\n$$\n"
 
 const unsafeimport = "" +
 	"cn\x00\x03v1\x01\vunsafe\x00\x05\r\rPointer\x00\x16\x00\t\x0fOffsetof\x00\x01" +
-	":\x00\x01\x16\x00\t\vSizeof\x00\x01:\x00\x01\x16\x00\t\rAlignof\x00\x01:\x00\x01\x16\x00\v\b\x00\v" +
-	"\x00\x01\x00\n$$\n"
+	":\x00\x01\x16\x00\t\vSizeof\x00\x01:\x00\x01\x16\x00\t\rAlignof\x00\x01:\x00\x01\x16\x00\v\b\v\x00" +
+	"\x01\x00\n$$\n"