Fix sysdeps/ia64/fpu/libm-symbols.h for inclusion in testcases.

Fix sysdeps/ia64/fpu/libm-symbols.h for inclusion in testcases.

IA64 builds in math/ use "-include libm-symbols.h".  This breaks the
signgam tests, which rely on undefining _LIBC and feature test macros
and defining _ISOMAC before including system headers to get them to
provide only ISO C declarations (libm-symbols.h ending up indirectly
including headers which result in this breaking); similarly, it breaks
C++ tests as some headers included from libm-symbols.h are not ready
for inclusion in C++ code.  This patch disables the contents of
libm-symbols.h if __STRICT_ANSI__ or __cplusplus are defined to avoid
this problem (this header can only test symbols defined before the
source file is processed, so can't test _ISOMAC because that's defined
within the source file, after this header is included).

Tested (compilation only) for ia64.

	* sysdeps/ia64/fpu/libm-symbols.h: Make contents conditional on
	[!__STRICT_ANSI__ && !__cplusplus].