libstdc++: Fix symbol version conflict in linker script

The change in r11-4748-50b840ac causes
a build error on Solaris, due to the new explicit instantiation matching
patterns for two different symbol versions.

libstdc++-v3/ChangeLog:

	* config/abi/pre/gnu.ver (GLIBCXX_3.4.21): Tighten up patterns
	for basic_stringbuf that refer to __xfer_bufptrs.