* double-conversion: enable for nios2

Nios2 supports double conversion, tested using qemu:

./main || echo "correct"
correct
uname -a
Linux buildroot 4.19.16 #4 Sat Aug 3 14:46:48 CEST 2019 nios2 GNU/Linux

Solves build error

In file included from double-conversion.h:42:0,
                 from number_decimalquantity.cpp:19:
double-conversion-utils.h:119:2: error: #error Target architecture was not detected as supported by Double-Conversion.
 #error Target architecture was not detected as supported by Double-Conversion.

detected by buildroot autobuilders:
http://autobuild.buildroot.net/results/91e/91eaec34708d91f8a05af189243be0b7cabce31b/

Patch sent upstream: https://github.com/unicode-org/icu/pull/725
Bug report: https://unicode-org.atlassian.net/browse/ICU-20751



[Bernd: Fixed path]
Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
[Added Changelog entry]
Signed-off-by: Peter Seiderer <ps.report@gmx.net>

* double-conversion: enable for xtensa

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
---
Note:

Double-conversion tested with:

        $ cat div.c
double Div_double(double x, double y) { return x / y; }

        $ cat main.c
double Div_double(double x, double y);

int main(int argc, char** argv) {
  double result = Div_double(89255.0, 1e22);
  if (result == 89255e-22) {
    printf("correct result %e\n", result);
    return 1;
  } else {
    printf("wrong result %e\n", result);
    return 0;
  }
}

Tested for xtensa (using qemu_xtensa_lx60_defconfig plus BR2_PACKAGE_HOST_QEMU_LINUX_USER_MODE):

        $ ./host/bin/xtensa-buildroot-linux-uclibc-gcc -c div.c
        $ ./host/bin/xtensa-buildroot-linux-uclibc-gcc -c main.c
        $ ./host/bin/xtensa-buildroot-linux-uclibc-gcc -o main main.o div.o

        $ ./host/bin/qemu-xtensa -L staging main
correct result 8.925500e-18