[PR build/24886] disable glibc mcheck support

[PR build/24886] disable glibc mcheck support

This patch drops gdb's configury support for glibc's mcheck function.
It has been observed to cause false abort()s, because it is
thread-unsafe yet interposes every malloc/free operation.  So if any
library transitively used by gdb also uses threads, then these
functions can easily corrupt their own checking data.  These days, gcc
ASAN and valgrind provide high quality checking, and mcheck is
apparently itself being slowly deprecated.

So, let's stop linking to it.  Attached patch drops the
autoconf/Makefile machinery for both gdb and gdbserver.  No
testsuite-visible impact.  IMHO not worth mentioning in NEWS.

See also: https://sourceware.org/bugzilla/show_bug.cgi?id=9939

gdb/ChangeLog

        PR build/24886
        * configure.ac: Drop enable-libmcheck support.
        * configure, config.in: Rebuild.
        * libmcheck.m4: Remove.
        * acinclude.m4: Don't include it.
        * Makefile.in: Don't distribute it.
        * top.c (print_gdb_configuration): Don't mention it.

gdb/gdbserver/ChangeLog

        PR build/24886
        * configure.ac: Drop enable-libmcheck support.
        * configure, config.in: Rebuild.
        * acinclude.m4: Don't include it.