Fix Wundef warning for __cplusplus

All current uses of the __cplusplus macro only check if it is defined
or not.  Fix this #if to use $ifdef like the rest of the code.
Fix -Wmaybe-uninitialized warning in xdr.c

While we're at fixing build warnings, here's one unnecessary warning
that can be fixed fairly easily.  The SIZE variable is never actually
use uninitialized, but the compiler cannot make that out and thinks
(correctly) that there is a potential for accessing SIZE without
initializing it.  Make this safe by initializing SIZE to 0.

Tested on x86_64.
Fix -Wundef warnings for SHARED

The definition of SHARED is tested with #ifdef pretty much everywhere
apart from these few places. The tlsdesc.c code seems to be copy and
pasted to a few architectures and there is one instance in the hppa
startup code.

ChangeLog:

2014-07-09  Will Newton  <will.newton@linaro.org>

	* sysdeps/aarch64/tlsdesc.c (_dl_unmap): Test SHARED with #ifdef.
	* sysdeps/arm/tlsdesc.c (_dl_unmap): Likewise.
	* sysdeps/i386/tlsdesc.c (_dl_unmap): Likewise.
	* sysdeps/x86_64/tlsdesc.c (_dl_unmap): Likewise.
	* sysdeps/hppa/start.S (_start): Likewise.
Update i386 libm test ULPs
Fix missing newline in test output