Replace 1L with (mp_limb_t) 1

X86-64 and x32 use sysdeps/i386/ldbl2mpn.c.  res_ptr is a pointer
to mp_limb_t, which is long for i386 and x86-64 and long long for
x32.  On x32, I got

../sysdeps/x86_64/../i386/ldbl2mpn.c: In function ‘__mpn_extract_long_double’:
../sysdeps/x86_64/../i386/ldbl2mpn.c:72:4: error: left shift count >= width of type [-Werror]
    res_ptr[N - 1] &= ~(1L << ((LDBL_MANT_DIG - 1) % BITS_PER_MP_LIMB));
    ^
cc1: all warnings being treated as errors

This patch replaces 1L with (mp_limb_t) 1.  Verified on x32, i686 and
x86-64 with GCC 4.8.3.