libstdc++: Fix preprocessor condition in linker script [PR108969]

The linker script is preprocessed with $(top_builddir)/config.h not the
include/$target/bits/c++config.h version, which means that configure
macros do not have the _GLIBCXX_ prefix yet.

The _GLIBCXX_SYMVER_GNU and _GLIBCXX_SHARED checks are redundant,
because the gnu.ver file is only used for _GLIBCXX_SYMVER_GNU and the
linker script is only used for the shared library. Remove those.

libstdc++-v3/ChangeLog:

	PR libstdc++/108969
	* config/abi/pre/gnu.ver: Fix preprocessor condition.