[X86][Draft] Disable long double type for -mno-x87 option

[asavonic]

Summary

This patch attempts to fix a compiler crash that occurs when long double type
is used with -mno-x87 compiler option.

The option disables x87 target feature, which in turn disables x87 registers, so
CG cannot select them for x86_fp80 LLVM IR type. Long double is lowered as
x86_fp80 for some targets, so it leads to a crash.

The option seems to contradict the SystemV ABI, which requires long double to
be represented as a 80-bit floating point, and it also requires to use x87 registers.

[asavonic]

I'm not sure that this is the right approach, but I wanted to get feedback on
how the issue should be fixed. Currently, the compiler crashes on almost any
code with long double and -mno-x87 (excluding cases where CG does not properly disable x87):

long double foo(long double x, long double y)
{
  long double z = x + y;
  if (z < 0.0)
    return z;
  else
    return 0.0;
}