Fix test-as-const-jmp_buf-ssp.c generation on gnu-i386

hurd's jmp_buf-ssp.sym does not define any symbol.
scripts/gen-as-const.py currently was emitting an empty line in that
case, and the gawk invocation was prepending "asconst_" to it, ending up
with:

.../build/glibc/setjmp/test-as-const-jmp_buf-ssp.c:1:2: error: expected « = », « , », « ; », « asm » or
« __attribute__ » at end of input
    1 |  asconst_
      |  ^~~~~~~~

        * scripts/gen-as-const.py (main): Avoid emitting empty line when
        there is no element in `consts'.