cmd/5l: add PT_PAX_FLAGS ELF header

Although I don't use PAX enabled ARM kernels, PAX
does have support for ARM, so we're better off add
PT_PAX_FLAGS now in case people use PAX kernels.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/6453092

src/cmd/5l/asm.c

@@ -963,6 +963,11 @@ asmb(void)
 		ph->flags = PF_W+PF_R;
 		ph->align = 4;
 
+		ph = newElfPhdr();
+		ph->type = PT_PAX_FLAGS;
+		ph->flags = 0x2a00; // mprotect, randexec, emutramp disabled
+		ph->align = 4;
+
 		sh = newElfShstrtab(elfstr[ElfStrShstrtab]);
 		sh->type = SHT_STRTAB;
 		sh->addralign = 1;