arc: build libgcc in U-Boot

This way we may have very limited set of functions implemented so we
save some space.

Also it allows us to build U-Boot for any ARC core with the same one
toolchain because we don't rely on pre-built libgcc.

For example:
 * we may use little-endian toolchain but build U-Boot for ether
endianess
 * we may use non-multilibbed uClibc toolchain but build U-Boot for
whatever ARC CPU flavour that current GCC supports

Private libgcc built from generic C implementation contributes only 144
bytes to .text section so we don't see significant degradation of size:
--->8---
$ arc-linux-size u-boot.libgcc-prebuilt
   text	   data	    bss	    dec	    hex	filename
 222217	  24912	 214820	 461949	  70c7d	u-boot.libgcc-prebuilt

$ arc-linux-size u-boot.libgcc-private
   text	   data	    bss	    dec	    hex	filename
 222361	  24912	 214820	 462093	  70d0d	u-boot.libgcc-private
--->8---

Also I don't notice visible performance degradation compared to
pre-built libgcc (where at least "*div*" functions are had-written in
assembly) on typical operations of downloading 10Mb uImage over TFTP and
bootm.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>

arch/arc/lib/libgcc2.c

@@ -0,0 +1,161 @@
+/*
+ * Copyright (C) 1989-2013 Free Software Foundation, Inc.
+ *
+ * SPDX-License-Identifier:	GPL-2.0+
+ */
+
+#include "libgcc2.h"
+
+DWtype
+__ashldi3(DWtype u, shift_count_type b)
+{
+	if (b == 0)
+		return u;
+
+	const DWunion uu = {.ll = u};
+	const shift_count_type bm = W_TYPE_SIZE - b;
+	DWunion w;
+
+	if (bm <= 0) {
+		w.s.low = 0;
+		w.s.high = (UWtype)uu.s.low << -bm;
+	} else {
+		const UWtype carries = (UWtype) uu.s.low >> bm;
+
+		w.s.low = (UWtype)uu.s.low << b;
+		w.s.high = ((UWtype)uu.s.high << b) | carries;
+	}
+
+	return w.ll;
+}
+
+DWtype
+__ashrdi3(DWtype u, shift_count_type b)
+{
+	if (b == 0)
+		return u;
+
+	const DWunion uu = {.ll = u};
+	const shift_count_type bm = W_TYPE_SIZE - b;
+	DWunion w;
+
+	if (bm <= 0) {
+		/* w.s.high = 1..1 or 0..0 */
+		w.s.high = uu.s.high >> (W_TYPE_SIZE - 1);
+		w.s.low = uu.s.high >> -bm;
+	} else {
+		const UWtype carries = (UWtype) uu.s.high << bm;
+
+		w.s.high = uu.s.high >> b;
+		w.s.low = ((UWtype)uu.s.low >> b) | carries;
+	}
+
+	return w.ll;
+}
+
+DWtype
+__lshrdi3(DWtype u, shift_count_type b)
+{
+	if (b == 0)
+		return u;
+
+	const DWunion uu = {.ll = u};
+	const shift_count_type bm = W_TYPE_SIZE - b;
+	DWunion w;
+
+	if (bm <= 0) {
+		w.s.high = 0;
+		w.s.low = (UWtype)uu.s.high >> -bm;
+	} else {
+		const UWtype carries = (UWtype)uu.s.high << bm;
+
+		w.s.high = (UWtype)uu.s.high >> b;
+		w.s.low = ((UWtype)uu.s.low >> b) | carries;
+	}
+
+	return w.ll;
+}
+
+unsigned long
+udivmodsi4(unsigned long num, unsigned long den, int modwanted)
+{
+	unsigned long bit = 1;
+	unsigned long res = 0;
+
+	while (den < num && bit && !(den & (1L<<31))) {
+		den <<= 1;
+		bit <<= 1;
+	}
+
+	while (bit) {
+		if (num >= den) {
+			num -= den;
+			res |= bit;
+		}
+		bit >>= 1;
+		den >>= 1;
+	}
+
+	if (modwanted)
+		return num;
+
+	return res;
+}
+
+long
+__divsi3(long a, long b)
+{
+	int neg = 0;
+	long res;
+
+	if (a < 0) {
+		a = -a;
+		neg = !neg;
+	}
+
+	if (b < 0) {
+		b = -b;
+		neg = !neg;
+	}
+
+	res = udivmodsi4(a, b, 0);
+
+	if (neg)
+		res = -res;
+
+	return res;
+}
+
+long
+__modsi3(long a, long b)
+{
+	int neg = 0;
+	long res;
+
+	if (a < 0) {
+		a = -a;
+		neg = 1;
+	}
+
+	if (b < 0)
+		b = -b;
+
+	res = udivmodsi4(a, b, 1);
+
+	if (neg)
+		res = -res;
+
+	return res;
+}
+
+long
+__udivsi3(long a, long b)
+{
+	return udivmodsi4(a, b, 0);
+}
+
+long
+__umodsi3(long a, long b)
+{
+	return udivmodsi4(a, b, 1);
+}