cmd/ld: set ELF header flags for our Linux/ARM binary

To make it more compliant.
This won't affect the behavior of running on OABI-only kernels.

R=golang-dev, iant
CC=golang-dev
https://golang.org/cl/6475044

src/cmd/ld/elf.c

@@ -52,6 +52,9 @@ elfinit(void)
 		break;
 
 	// 32-bit architectures
+	case '5':
+		hdr.flags = 0x5000002; // has entry point, Version5 EABI
+		// fallthrough
 	default:
 		hdr.phoff = ELF32HDRSIZE;	/* Must be be ELF32HDRSIZE: first PHdr must follow ELF header */
 		hdr.shoff = ELF32HDRSIZE;	/* Will move as we add PHeaders */