libstdc++: Fix failing performance test

This test no longer builds because (file >> line) is ill-formed in
C++20, since r11-2581-g17abcc77341584 which implemented LWG 2499.

There's no reason to use a dynamically-allocated array when the length
is a small constant. We can just use an array on the stack.

libstdc++-v3/ChangeLog:

	* testsuite/performance/27_io/ifstream_extract_chars.cc: Use
	stack array instead of dynamic allocation.

Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>