Simplify math-svid-compat code.

Now there are no more assembly wrappers using _LIB_VERSION or
__kernel_standard, the math-svid-compat code can be slighly
simplified.  math-svid-compat.h no longer needs __ASSEMBLER__
conditionals, and the _LIB_VERSION variable no longer needs to be
built for static libm, since all references are now in C code that
includes math-svid-compat.h and so gets the macro definition of
_LIB_VERSION to _POSIX_ outside the compat case.  This patch makes
those cleanups.

Tested for x86_64, and with build-many-glibcs.py.

	* math/math-svid-compat.h [!__ASSEMBLER__]: Make code
	unconditional.
	* sysdeps/ieee754/s_lib_version.c [!defined SHARED]: Remove
	conditional code; define contents only for [LIBM_SVID_COMPAT].