PR middle-end/123826: __builtin_pow vs. errno.

This is my proposed solution to PR middle-end/123826.  Initially
I thought this would be a "one line change", adding a test for
flag_errno_math to gimple_expand_builtin_pow.  Unfortunately
this revealed a second later problem, where pow (with constant
arguments) was still getting evaluated at compile-time, even when
the result is known to overflow.

It's ancient history, but shortly after I added support for pow
as a builtin, I contributed code to evaluate it at compile-time
when the exponent is an integer constant.  Since then we now use
MPFR to evaluate libm math functions at compile-time.  However
the vestigial call to evaluate pow via real_powi still exists,
and gets invoked after do_mpfr_arg2/mpfr_pow correctly determines
that we shouldn't evaluate pow at compile-time.  This patch reorganizes
fold_const_pow paying attention to signaling NaNs (PR 61441)
and flag_errno_math.  Most importantly normal cases like pow(2.0,3.0)
and pow(3.0,0.5) still get evaluated at compile-time.

2026-02-03  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	PR middle-end/123826
	* tree-ssa-math-opts.cc (gimple_expand_builtin_pow): Add test
	for flag_errno_math.
	* fold-const-call.cc (fold_const_pow): Reorganize, eliminating
	call to real_powi, and letting do_mpfr_arg2 do all the heavy
	lifting.

gcc/testsuite/ChangeLog
	PR middle-end/123826
	* gcc.dg/errno-2.c: New test case.
	* gcc.dg/errno-3.c: Likewise.

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