6l, 8l: include ELF header in PT_LOAD mapping for text segment

Due to page boundary rounding, the header would have
been loaded as part of the text segment already, but this
change placates the "paxctl" tool on so-called hardened
Linux distributions (as if normal distributions weren't already
hard enough to use).

R=r
CC=golang-dev
https://golang.org/cl/954041

src/cmd/6l/asm.c

@@ -713,11 +713,11 @@ asmb(void)
 		ph = newElfPhdr();
 		ph->type = PT_LOAD;
 		ph->flags = PF_X+PF_R;
-		ph->vaddr = va;
-		ph->paddr = va;
-		ph->off = fo;
-		ph->filesz = w;
-		ph->memsz = w;
+		ph->vaddr = va - fo;
+		ph->paddr = va - fo;
+		ph->off = 0;
+		ph->filesz = w + fo;
+		ph->memsz = w + fo;
 		ph->align = INITRND;
 
 		fo = rnd(fo+w, INITRND);

src/cmd/8l/asm.c

@@ -829,11 +829,11 @@ asmb(void)
 		ph = newElfPhdr();
 		ph->type = PT_LOAD;
 		ph->flags = PF_X+PF_R;
-		ph->vaddr = va;
-		ph->paddr = va;
-		ph->off = fo;
-		ph->filesz = w;
-		ph->memsz = w;
+		ph->vaddr = va - fo;
+		ph->paddr = va - fo;
+		ph->off = 0;
+		ph->filesz = w + fo;
+		ph->memsz = w + fo;
 		ph->align = INITRND;
 
 		fo = rnd(fo+w, INITRND);