nexttowardl for ppc64el generates wrong results

[Hubert Tong]

On PPC, the non-IEEE 128-bit long double is represented as a pair of doubles. This means that for 1.L, the next representable value in the direction of 2.L should be 1.L + DBL_EPSILON. The nexttowardl function does not seem to produce such a result.

To reproduce, compile and link the following C program and run the resulting executable. The actual output and the expected output is provided below.

### SOURCE (<stdin>):
long double nexttowardl(long double, long double);
int printf(const char *, ...);

int main(void) {
  long double eps = nexttowardl(0.L, 1.L);
  long double ans = nexttowardl(1.L, 2.L);
  long double ref = 1.L + eps;
  typedef unsigned char *ucp;
  for (ucp p = (ucp)&ref; p < (ucp)&1[&ref]; ++p) {
    printf("%02hhx", *p);
  }
  printf("\n");
  for (ucp p = (ucp)&ans; p < (ucp)&1[&ans]; ++p) {
    printf("%02hhx", *p);
  }
  printf("\n");
}

### COMPILER INVOCATION:
gcc -xc - -lm

### RUN OUTPUT:
000000000000f03f0100000000000000
000000000000f03f0000000000006039

### EXPECTED OUTPUT:
000000000000f03f0100000000000000
000000000000f03f0100000000000000

### VERSION INFO:
> lsb_release -rd
Description: Ubuntu 16.04.2 LTS
Release: 16.04
Return: 0x00:0

> apt-cache policy libc6
libc6:
  Installed: 2.23-0ubuntu7
  Candidate: 2.23-0ubuntu11
  Version table:
     2.23-0ubuntu11 500
        500 http://ports.ubuntu.com/ubuntu-ports xenial-updates/main ppc64el Packages
     2.23-0ubuntu10 500
        500 http://ports.ubuntu.com/ubuntu-ports xenial-security/main ppc64el Packages
 *** 2.23-0ubuntu7 100
        100 /var/lib/dpkg/status
     2.23-0ubuntu3 500
        500 http://ports.ubuntu.com/ubuntu-ports xenial/main ppc64el Packages
Return: 0x00:0