mips dynamic linker support

not heavily tested, but the basics are working. the basic concept is
that the dynamic linker entry point code invokes a pure-PIC (no global
accesses) C function in reloc.h to perform the early GOT relocations
needed to make the dynamic linker itself functional, then invokes
__dynlink like on other archs. since mips uses some ugly arch-specific
hacks to optimize relocating the GOT (rather than just using the
normal DT_REL[A] tables like on other archs), the dynamic linker has
been modified slightly to support calling arch-specific relocation
code in reloc.h.

most of the actual mips-specific behavior was developed by reading the
output of readelf on libc.so and simple executable files. i could not
find good reference information on which relocation types need to be
supported or their semantics, so it's possible that some legitimate
usage cases will not work yet.

src/ldso/dynlink.c

@@ -487,6 +487,9 @@ static void reloc_all(struct dso *p)
 	for (; p; p=p->next) {
 		if (p->relocated) continue;
 		decode_vec(p->dynv, dyn, DYN_CNT);
+#ifdef NEED_ARCH_RELOCS
+		do_arch_relocs(p, head);
+#endif
 		do_relocs(p, (void *)(p->base+dyn[DT_JMPREL]), dyn[DT_PLTRELSZ],
 			2+(dyn[DT_PLTREL]==DT_RELA));
 		do_relocs(p, (void *)(p->base+dyn[DT_REL]), dyn[DT_RELSZ], 2);

src/ldso/mips/start.s

@@ -0,0 +1,46 @@
+.hidden _DYNAMIC
+.hidden __reloc_self
+.set noreorder
+.set nomacro
+.global _start
+.type _start,@function
+_start:
+	move $fp, $0
+
+	bgezal $0, 1f
+	nop
+2:	.gpword 2b
+	.gpword _DYNAMIC
+	.gpword __reloc_self
+1:	lw $gp, 0($ra)
+	subu $gp, $ra, $gp
+
+	lw $4, 0($sp)
+	addiu $5, $sp, 4
+	lw $6, 4($ra)
+	addu $6, $6, $gp
+	addiu $7, $gp, -0x7ff0
+	subu $sp, $sp, 16
+	lw $25, 8($ra)
+	add $25, $25, $gp
+	jalr $25
+	nop
+
+	lw $25, %call16(__dynlink)($gp)
+	lw $4, 16($sp)
+	addiu $5, $sp, 20
+	jalr $25
+	nop
+
+	add $sp, $sp, 16
+	li $6, -1
+1:	lw $4, ($sp)
+	lw $5, 4($sp)
+	bne $5, $6, 2f
+	nop
+	addu $sp, $sp, 4
+	addu $4, $4, -4
+	b 1b
+	nop
+2:	sw $4, ($sp)
+	jr $2