Integrate PR 12649's race detector directly in the testsuite machinery

Integrate PR 12649's race detector directly in the testsuite machinery

This integrates Jan Kratochvil's nice race reproducer from PR
testsuite/12649 into the testsuite infrustructure directly.

With this, one only has to do either 'make check-read1' or 'make check
READ1="1"' to preload the read1.so library into expect.

Currently only enabled for glibc/GNU systems, and if
build==host==target.

gdb/testsuite/ChangeLog:

	* Makefile.in (EXTRA_RULES, CC): New variables, get from
	configure.
	(EXPECT): Handle READ1 being set.
	(all): Depend on EXTRA_RULES.
	(check-read1, expect-read1, read1.so, read1): New rules.
	* README (Testsuite Parameters): Document the READ1 make variable.
	(Race detection): New section.
	* configure: Regenerate.
	* configure.ac: If build==host==target, and running under a
	GNU/glibc system, add read1 to the extra Makefile rules.
	(EXTRA_RULES): AC_SUBST it.
	* lib/read1.c: New file.

gdb/ChangeLog:

	* Makefile.in (check-read1): New rule.