contrib: Fill in HOST{CC,CFLAGS,CXX,CXXFLAGS} in test_installed

gcc/Makefile.in since my r0-60234 change fills in HOSTCC and HOSTCFLAGS
in site.exp and since r8-671 also HOSTCXX and HOSTCXXFLAGS.
If those variables aren't set, we get errors like:
/usr/src/gcc/contrib/test_installed --without-g++ --without-gfortran --without-objc struct-layout-1.exp
...
ERROR: tcl error sourcing /usr/src/gcc/gcc/testsuite/gcc.dg/compat/struct-layout-1.exp.
ERROR: tcl error code TCL LOOKUP VARNAME HOSTCC
ERROR: can't read "HOSTCC": no such variable
    while executing
"remote_exec build "$HOSTCC $HOSTCFLAGS $generator_cmd""
    (file "/usr/src/gcc/gcc/testsuite/gcc.dg/compat/struct-layout-1.exp" line 96)
    invoked from within
"source /usr/src/gcc/gcc/testsuite/gcc.dg/compat/struct-layout-1.exp"
    ("uplevel" body line 1)
    invoked from within
"uplevel #0 source /usr/src/gcc/gcc/testsuite/gcc.dg/compat/struct-layout-1.exp"
    invoked from within
"catch "uplevel #0 source $test_file_name" msg"

(similarly in g++ or gfortran) struct-layout-1.exp.  One doesn't need to
test specially for just struct-layout-1.exp alone, just not using any arg
will trigger it as well, just later.

The following patch fills it in as cc and c++ with empty flags to compile
those, I believe that is what e.g. make uses by default, so it should be a
reasonable default.  We IMHO shouldn't default to GCC_UNDER_TEST because
that might be a cross-compiler etc.

2024-02-05  Jakub Jelinek  <jakub@redhat.com>

	* test_installed: Fill in HOSTCC, HOSTCXX, HOSTCFLAGS and
	HOSTCXXFLAGS.

contrib/test_installed

@@ -114,6 +114,10 @@ set GCC_UNDER_TEST "${GCC_UNDER_TEST-${prefix}${prefix+/bin/}gcc}"
 set GXX_UNDER_TEST "${GXX_UNDER_TEST-${prefix}${prefix+/bin/}g++}"
 set GFORTRAN_UNDER_TEST "${GFORTRAN_UNDER_TEST-${prefix}${prefix+/bin/}gfortran}"
 set OBJC_UNDER_TEST "${OBJC_UNDER_TEST-${prefix}${prefix+/bin/}gcc}"
+set HOSTCC "${HOSTCC-cc}"
+set HOSTCXX "${HOSTCXX-c++}"
+set HOSTCFLAGS ""
+set HOSTCXXFLAGS ""
 EOF
 if test x${target} != x; then
   echo "set target_triplet $target" >> site.exp