new mostly-C crt1 implementation

the only immediate effect of this commit is enabling PIE support on
some archs that did not previously have any Scrt1.s, since the
existing asm files for crt1 override this C code. so some of the
crt_arch.h files committed are only there for the sake of documenting
what their archs "would do" if they used the new C-based crt1.

the expectation is that new archs should use this new system rather
than using heavy asm for crt1. aside from being easier and less
error-prone, it also ensures that PIE support is available immediately
(since Scrt1.o is generated from the same C source, using -fPIC)
rather than having to be added as an afterthought in the porting
process.

arch/mips/crt_arch.h

@@ -0,0 +1,21 @@
+__asm__("\n\
+.set push\n\
+.set noreorder\n\
+.global __start\n\
+.global _start\n\
+.type   __start, @function\n\
+.type   _start, @function\n\
+__start:\n\
+_start:\n\
+	bal 1f \n\
+	move $fp, $0 \n\
+2:	.gpword 2b \n\
+1:	lw $gp, 0($ra) \n\
+	subu $gp, $ra, $gp \n\
+	move $4, $sp \n\
+	subu $sp, $sp, 16 \n\
+	and $sp, $sp, -8 \n\
+	lw $25, %call16(__cstart)($gp) \n\
+	jalr $25 \n\
+	nop \n\
+.set pop");