tile: improve detection for missing -mcmodel=large support

The existing test avoided passing -mcmodel=large if the compiler didn't
support it.  However, we need to test not just the compiler support, but
also the toolchain (as and ld) support, so make the test more complete.
In addition, we have to avoid using the hwN_plt() assembly operators if
that support is missing, so guard the uses with #ifdef NO_PLT_PCREL.

This allows us to properly build glibc with the current community
binutils, which doesn't yet have the PC-relative PLT operator support.
The -mcmodel=large support is in gcc 4.8, but the toolchain support
won't be present in the community until binutils 2.24.

(cherry picked from commit 86bd05fbc8b3a635148f6a7d8b4fb89c9a524e58)
tile: default to little-endian in bits/endian.h

This turns out to be helpful when doing a from-scratch cross-compile of
gcc and glibc, since you can then do "make install-headers" in glibc
even before you have a functioning tile gcc.

(cherry picked from commit ad36ba2bd67398edefe31aa039090912f76bffce)