* flatbuffers: Move EndianSwap template to flatbuffers/base.h

Clang complains
call to function 'EndianSwap' that is neither visible in the template definition nor found by argument-dependent lookup
     return EndianSwap(t);

This seems to be due to limitation of two-phase lookup of dependent names in template definitions

Its not being found using associated namespaces therefore
it has to be made visible at the template definition site as well
Signed-off-by: Khem Raj <raj.khem@gmail.com>

* use __builtin_bswap16 when building with clang

clang pretends to be gcc 4.2.0 and therefore the code does
not use __builtin_bswap16 but tries to synthesize it
Signed-off-by: Khem Raj <raj.khem@gmail.com>