builtins: Guard builtins.cc against HUGE_VAL and NAN definitions

builtins: Guard builtins.cc against HUGE_VAL and NAN definitions

trunk bootstrap recently broke on Solaris like this:

/vol/gcc/src/hg/master/local/gcc/builtins.cc:2104:8: error: pasting
"CFN_BUILT_IN_" and "(" does not give a valid preprocessing token
 2104 |   case CFN_BUILT_IN_##MATHFN:   \
      |        ^~~~~~~~~~~~~
/vol/gcc/src/hg/master/local/gcc/builtins.cc:2112:3: note: in expansion of
macro 'CASE_MATHFN'
 2112 |   CASE_MATHFN(MATHFN)    \
      |   ^~~~~~~~~~~
/vol/gcc/src/hg/master/local/gcc/builtins.cc:1967:5: note: in expansion of macro 'CASE_MATHFN_FLOATN'
 1967 |     CASE_MATHFN_FLOATN (HUGE_VAL)  \

and similarly for NAN.

It turns out this happens because <math.h> is included at some point,
which (in <iso/math_c99.h>) defines

While this only happpens on Solaris right now, the same issue would be
present on other targets when <math.h> gets included somehow.

To avoid this, this patch #undef's both macros.

Bootstrapped without regressions on i386-pc-solaris2.11 and
sparc-sun-solaris2.11.

2022-11-01  Rainer Orth  <ro@CeBiTec.Uni-Bielefeld.DE>

	gcc:
	* builtins.cc (mathfn_built_in_2): #undef HUGE_VAL, NAN.