[compiler-rt] Make builtins test pass when using i386 gcc as host compiler

Just-built-clang is used to compile the test, but the library is built
with gcc, so the usual 80-bit FPU vs 32-bit SSE mismatch makes the
floating computations not bitwise identical. Fixes PR32910, see there
for details.

This uses the same technique used in all the other *c3* tests, see in
particular mulsc3_test.c.

(It might be cleaner to add compareResultCF to fp_test.h to force the
floats into 32-bit in memory, but this is the less invasive fix.)

Differential Revision: https://reviews.llvm.org/D61684

llvm-svn: 360264