[builtins] Fix signed integer overflows in fp_fixint_impl.inc

When compiling the builtins with the undefined behavior sanitizer and running
testcases you end up with the following warning:

UBSan: fp_fixint_impl.inc:39:42: left shift of 8388608 by 40 places cannot be represented in type 'fixint_t' (aka 'long long')
UBSan: fp_fixint_impl.inc:39:17: signed integer overflow: -1 * -9223372036854775808 cannot be represented in type 'fixint_t' (aka 'long long')

This can be avoided by doing the shift and the multiplication in a matching
unsigned variant of the type.

The added test only trigger the intended signed overflow case when the builtins
are built with -D__SOFTFP__.

This was found in an out of tree target.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D159069