libstdc++-v3 testsuite: Call fesetround(FE_DOWNWARD) only if defined

Without this, for a typical soft-float target such as cris-elf, after
commit r12-7676-g5a4e208022e704 you'll see, in libstdc++.log:
...
FAIL: 20_util/from_chars/6.cc (test for excess errors)
Excess errors:
/home/hp/tmp/auto0321/gcc/libstdc++-v3/testsuite/20_util/from_chars/6.cc:33: error: 'FE_DOWNWARD' was not declared in this scope

UNRESOLVED: 20_util/from_chars/6.cc compilation failed to produce executable
...

It appears to be a side-effect of that commit changing the
way __cpp_lib_to_chars is defined.  (On the bright side,
./7.cc now passes since that commit.)

TFM, specifically fenv(3), says that "Each of the macros
FE_DIVBYZERO, FE_INEXACT, FE_INVALID, FE_OVERFLOW,
FE_UNDERFLOW is defined when the implementation supports
handling of the corresponding exception".

A git-grep shows that this was the only place using a FE_ macro
unconditionally.

libstdc++-v3:
	* testsuite/20_util/from_chars/6.cc (test01) [FE_DOWNWARD]:
	Conditionalize call to fesetround.