add support for init/finit (constructors and destructors)

this is mainly in hopes of supporting c++ (not yet possible for other
reasons) but will also help applications/libraries which use (and more
often, abuse) the gcc __attribute__((__constructor__)) feature in "C"
code.

x86_64 and arm versions of the new startup asm are untested and may
have minor problems.

crt/arm/crt1.s

@@ -1,13 +1,16 @@
+.weak _init
+.weak _fini
 .global _start
 _start:
 	mov fp,#0
 	mov lr,#0
 	ldr a2,[sp],#4
 	mov a3,sp
-	mov a4,#0
+	ldr a4,=_fini
 	str fp,[sp,#-4]!
 	str a1,[sp,#-4]!
-	str fp,[sp,#-4]!
+	str a4,[sp,#-4]!
+	ldr a4,=_init
 	ldr a1,=main
 	bl __libc_start_main
 1:	b 1b

crt/arm/crti.s

@@ -0,0 +1,7 @@
+.section .init
+.global _init
+_init:
+
+.section .fini
+.global _fini
+_fini:

crt/arm/crtn.s

@@ -0,0 +1,9 @@
+.section .init
+	tst lr,#1
+	moveq pc,lr
+	bx lr
+
+.section .fini
+	tst lr,#1
+	moveq pc,lr
+	bx lr