Please support other architectures

[Impact]

libdfp is currently only built for ppc64el and s390x. This causes gcc compilation to fail on other architectures in certain cases.

[Test Case]

$ echo "int main(void) {
_Decimal64 dvalue = 0.4dd;
__int128 ivalue = (__int128)dvalue;
return (int)ivalue;
}" | gcc -x c -
/tmp/cc97Mfy0.o: In function `main':
:(.text+0x1c): undefined reference to `__bid_fixddti'
collect2: error: ld returned 1 exit status

# bid_fixddti is part of the libdfp API: https://github.com/libdfp/libdfp/blob/master/sysdeps/bid/libdfp.abilist

$ echo "int main(void) {
_Decimal64 dvalue = 0.4dd;
__int128 ivalue = (__int128)dvalue;
return (int)ivalue;
}" | gcc -x c -ldfp -
/usr/bin/ld: cannot find -ldfp
collect2: error: ld returned 1 exit status

$ sudo apt install libdfp-dev
Reading package lists... Done
Building dependency tree
Reading state information... Done
E: Unable to locate package libdfp-dev

$ rmadison libdfp-dev
 libdfp-dev | 1.0.12-0ubuntu1 | xenial/universe | ppc64el, s390x
 libdfp-dev | 1.0.13-1 | bionic/universe | ppc64el, s390x
 libdfp-dev | 1.0.13-1 | cosmic/universe | ppc64el, s390x
 libdfp-dev | 1.0.13-1 | disco/universe | ppc64el, s390x

[Regression Potential]

This could regress existing packages if changes are required in order to make it work on new architectures.