build: change GO386=sse to GO386=sse2

sse2 is a more precise description of the requirement,
and it matches what people will see in, for example
        grep sse2 /proc/cpuinfo # linux
        sysctl hw.optional.sse2 # os x

R=golang-dev, dsymonds, iant
CC=golang-dev
https://golang.org/cl/7057050

src/cmd/dist/a.h

@@ -123,7 +123,7 @@ void	runv(Buf *b, char *dir, int mode, Vec *argv);
 void	bgrunv(char *dir, int mode, Vec *argv);
 void	bgwait(void);
 bool	streq(char*, char*);
-bool	cansse(void);
+bool	cansse2(void);
 void	writefile(Buf*, char*, int);
 void	xatexit(void (*f)(void));
 void	xexit(int);

src/cmd/dist/build.c

@@ -105,8 +105,8 @@ init(void)
 
 	xgetenv(&b, "GO386");
 	if(b.len == 0) {
-		if(cansse())
-			bwritestr(&b, "sse");
+		if(cansse2())
+			bwritestr(&b, "sse2");
 		else
 			bwritestr(&b, "387");
 	}

src/cmd/dist/plan9.c

@@ -759,7 +759,7 @@ xtryexecfunc(void (*f)(void))
 }
 
 bool
-cansse(void)
+cansse2(void)
 {
 	// if we had access to cpuid, could answer this question
 	// less conservatively.

src/cmd/dist/unix.c

@@ -780,7 +780,7 @@ __cpuid(int dst[4], int ax)
 }
 
 bool
-cansse(void)
+cansse2(void)
 {
 	int info[4];
 	

src/cmd/dist/windows.c

@@ -987,7 +987,7 @@ cpuid(int dst[4], int ax)
 }
 
 bool
-cansse(void)
+cansse2(void)
 {
 	int info[4];
 	

src/cmd/gc/lex.c

@@ -217,7 +217,6 @@ main(int argc, char *argv[])
 	goroot = getgoroot();
 	goos = getgoos();
 	goarch = thestring;
-	use_sse = strcmp(getgo386(), "sse") == 0;
 	
 	setexp();
 
@@ -261,6 +260,9 @@ main(int argc, char *argv[])
 
 	flagparse(&argc, &argv, usage);
 
+	if(argc < 1)
+		usage();
+
 	if(flag_race) {
 		racepkg = mkpkg(strlit("runtime/race"));
 		racepkg->name = "race";
@@ -273,8 +275,15 @@ main(int argc, char *argv[])
 	if(debug['l'] <= 1)
 		debug['l'] = 1 - debug['l'];
 
-	if(argc < 1)
-		usage();
+	if(thechar == '8') {
+		p = getgo386();
+		if(strcmp(p, "387") == 0)
+			use_sse = 0;
+		else if(strcmp(p, "sse2") == 0)
+			use_sse = 1;
+		else
+			sysfatal("unsupported setting GO386=%s", p);
+	}
 
 	pathname = mal(1000);
 	if(getwd(pathname, 999) == 0)