LoongArch: Stop -mfpu from silently breaking ABI [PR109000]

LoongArch: Stop -mfpu from silently breaking ABI [PR109000]

In the toolchain convention, we describe -mfpu= as:

"Selects the allowed set of basic floating-point instructions and
registers. This option should not change the FP calling convention
unless it's necessary."

Though not explicitly stated, the rationale of this rule is to allow
combinations like "-mabi=lp64s -mfpu=64".  This will be useful for
running applications with LP64S/F ABI on a double-float-capable
LoongArch hardware and using a math library with LP64S/F ABI but native
double float HW instructions, for a better performance.

And now a case in Linux kernel has again proven the usefulness of this
kind of combination.  The AMDGPU DCN kernel driver needs to perform some
floating-point operation, but the entire kernel uses LP64S ABI.  So the
translation units of the AMDGPU DCN driver need to be compiled with
-mfpu=64 (the kernel lacks soft-FP routines in libgcc), but -mabi=lp64s
(or you can't link it with the other part of the kernel).

Unfortunately, currently GCC uses TARGET_{HARD,SOFT,DOUBLE}_FLOAT to
determine the floating calling convention.  This causes "-mfpu=64"
silently allow using $fa* to pass parameters and return values EVEN IF
-mabi=lp64s is used.  To make things worse, the generated object file
has SOFT-FLOAT set in the eflags field so the linker will happily link
it with other LP64S ABI object files, but obviously this will lead to
bad results at runtime.  And for now all loongarch64 CPU models (-march
settings) implies -mfpu=64 on by default, so the issue makes a single
"-mabi=lp64s" option basically broken (fortunately most projects for eg
the Linux kernel have used -msoft-float which implies both -mabi=lp64s
and -mfpu=none as we've recommended in the toolchain convention doc).

The fix is simple: use TARGET_*_FLOAT_ABI instead.

I consider this a bug fix: the behavior difference from the toolchain
convention doc is a bug, and generating object files with SOFT-FLOAT
flag but parameters/return values passed through FPRs is definitely a
bug.

Bootstrapped and regtested on loongarch64-linux-gnu.  Ok for trunk and
release/gcc-12 branch?

gcc/ChangeLog:

	PR target/109000
	* config/loongarch/loongarch.h (FP_RETURN): Use
	TARGET_*_FLOAT_ABI instead of TARGET_*_FLOAT.
	(UNITS_PER_FP_ARG): Likewise.

gcc/testsuite/ChangeLog:

	PR target/109000
	* gcc.target/loongarch/flt-abi-isa-1.c: New test.
	* gcc.target/loongarch/flt-abi-isa-2.c: New test.
	* gcc.target/loongarch/flt-abi-isa-3.c: New test.
	* gcc.target/loongarch/flt-abi-isa-4.c: New test.