[compiler-rt] Avoid signed overflow in floatdidf.c and floatdisf.c

When compiling compiler-rt with -fsanitize=undefined and running testcases you
end up with the following warning:

UBSan: floatdisf.c:27:15: signed integer overflow: 9223372036854775807 - -1 cannot be represented in type 'di_int' (aka 'long long')

This can be avoided by doing the subtraction in a matching unsigned variant of
the type, given that the overflow is the expected result of the subtraction.

The same kind of pattern exists in floatdidf.c

This was found in an out of tree target.

Reviewed By: phosek

Differential Revision: https://reviews.llvm.org/D146135