cmd/dist: support for NetBSD/ARM

1. when executing a unsupported VFP instruction, the NetBSD kernel somehow
doesn't report SIGILL, and instead just spin and spin, we add a alarm(2)
to detect this case (albeit this is a kernel bug).
2. NetBSD/ARM's VFP11 support is not complete, so temporarily disable it.
3. The default gcc shipped with NetBSD-current mis-optimizes our code
at -O2, so lower the optimization level to -O1 on NetBSD/ARM.

R=dave, rsc
CC=golang-dev
https://golang.org/cl/7286044

src/cmd/dist/arm.c

@@ -17,6 +17,18 @@ static void useVFPv1(void);
 char *
 xgetgoarm(void)
 {
+#if defined(__NetBSD__) || defined(__FreeBSD__)
+	// NetBSD has buggy support for VFPv2 (incorrect inexact, 
+	// denormial, and NaN handling). When GOARM=6, some of our
+	// math tests fails on Raspberry Pi.
+	// Thus we return "5" here for safety, the user is free
+	// to override.
+	// Note: using GOARM=6 with cgo can trigger a kernel assertion
+	// failure and crash NetBSD/evbarm kernel.
+	// FreeBSD also have broken VFP support, so disable VFP also
+	// on FreeBSD.
+	return "5";
+#endif
 	if(xtryexecfunc(useVFPv3))
 		return "7";
 	else if(xtryexecfunc(useVFPv1))

src/cmd/dist/build.c

@@ -411,7 +411,13 @@ static char *proto_gccargs[] = {
 	"-fno-common",
 	"-ggdb",
 	"-pipe",
+#if defined(__NetBSD__) && defined(__arm__)
+	// GCC 4.5.4 (NetBSD nb1 20120916) on ARM is known to mis-optimize gc/mparith3.c
+	// Fix available at http://patchwork.ozlabs.org/patch/64562/.
+	"-O1",
+#else
 	"-O2",
+#endif
 };
 
 static Vec gccargs;

src/cmd/dist/unix.c

@@ -745,17 +745,24 @@ static void sigillhand(int);
 // xtryexecfunc tries to execute function f, if any illegal instruction
 // signal received in the course of executing that function, it will
 // return 0, otherwise it will return 1.
+// Some systems (notably NetBSD) will spin and spin when executing VFPv3
+// instructions on VFPv2 system (e.g. Raspberry Pi) without ever triggering
+// SIGILL, so we set a 1-second alarm to catch that case.
 int
 xtryexecfunc(void (*f)(void))
 {
 	int r;
 	r = 0;
 	signal(SIGILL, sigillhand);
+	signal(SIGALRM, sigillhand);
+	alarm(1);
 	if(sigsetjmp(sigill_jmpbuf, 1) == 0) {
 		f();
 		r = 1;
 	}
 	signal(SIGILL, SIG_DFL);
+	alarm(0);
+	signal(SIGALRM, SIG_DFL);
 	return r;
 }