cmd/ld: add PT_PAX_FLAGS ELF header

PAX systems are Linux systems that are more paranoid about memory permissions.
These flags tell them to relax when running Go binaries.

Fixes #47.

R=iant
CC=golang-dev
https://golang.org/cl/6326054

src/cmd/6l/asm.c

@@ -1082,6 +1082,11 @@ asmb(void)
 		ph->type = PT_GNU_STACK;
 		ph->flags = PF_W+PF_R;
 		ph->align = 8;
+		
+		ph = newElfPhdr();
+		ph->type = PT_PAX_FLAGS;
+		ph->flags = 0x2a00; // mprotect, randexec, emutramp disabled
+		ph->align = 8;
 
 		sh = newElfShstrtab(elfstr[ElfStrShstrtab]);
 		sh->type = SHT_STRTAB;

src/cmd/8l/asm.c

@@ -1127,6 +1127,11 @@ 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;

src/cmd/ld/elf.h

@@ -251,6 +251,7 @@ typedef struct {
 #define PT_LOPROC	0x70000000	/* First processor-specific type. */
 #define PT_HIPROC	0x7fffffff	/* Last processor-specific type. */
 #define PT_GNU_STACK	0x6474e551
+#define PT_PAX_FLAGS	0x65041580
 
 /* Values for p_flags. */
 #define PF_X		0x1		/* Executable. */