cmd/go, runtime: express armv5t architecture constraint differently

Instead of adding an -march=armv5t flag to the gcc command
line, the same effect is obtained with an ".arch armv5t"
pseudo op in the assembly file that uses armv5t instructions.

R=golang-dev, iant, dave
CC=golang-dev
https://golang.org/cl/14511044

src/cmd/go/build.go

@@ -1903,7 +1903,7 @@ func (b *builder) gccArchArgs() []string {
 	case "6":
 		return []string{"-m64"}
 	case "5":
-		return []string{"-marm", "-march=armv5t"} // not thumb
+		return []string{"-marm"} // not thumb
 	}
 	return nil
 }

src/pkg/runtime/cgo/gcc_arm.S

@@ -11,6 +11,13 @@
 #define EXT(s) s
 #endif
 
+/*
+ * Because the assembler might target an earlier revision of the ISA
+ * by default, we must explicitly specify the ISA revision to ensure
+ * BLX is recognized as a valid instruction.
+ */	
+.arch armv5t
+
 /*
  * void crosscall_arm2(void (*fn)(void), void (*setmg_gcc)(void *m, void *g), void *m, void *g)
  *