libquadmath: Use soft-fp for sqrtq finite positive arguments [PR114623]

sqrt should be 0.5ulp precise, but the current implementation is less
precise than that.
The following patch uses the soft-fp code (like e.g. glibc for x86) for it
if possible.  I didn't want to replicate the libgcc infrastructure for
choosing the right sfp-machine.h, so the patch just uses a single generic
implementation.  As the code is used solely for the finite positive arguments,
it shouldn't generate NaNs (so the exact form of canonical QNaN/SNaN is
irrelevant), and sqrt for these shouldn't produce underflows/overflows either,
for < 1.0 arguments it always returns larger values than the argument and for
> 1.0 smaller values than the argument.

2024-04-09  Jakub Jelinek  <jakub@redhat.com>

	PR libquadmath/114623
	* sfp-machine.h: New file.
	* math/sqrtq.c: Include from libgcc/soft-fp also soft-fp.h and quad.h
	if possible.
	(USE_SOFT_FP): Define in that case.
	(sqrtq): Use soft-fp based implementation for the finite positive
	arguments if possible.

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