libthread_db initialization changes related to upcoming glibc-2.34

libthread_db initialization changes related to upcoming glibc-2.34

This commit makes some adjustments to accomodate the upcoming
glibc-2.34 release.  Beginning with glibc-2.34, functionality formerly
contained in libpthread has been moved to libc.  For the time being,
libpthread.so still exists in the file system, but it won't show up in
ldd output and therefore won't be able to trigger initialization of
libthread_db related code.  E.g...

Fedora 34 / glibc-2.33.9000:

[kev@f34-2 gdb]$ ldd testsuite/outputs/gdb.threads/tls/tls
	linux-vdso.so.1 (0x00007ffcf94fa000)
	libstdc++.so.6 => /lib64/libstdc++.so.6 (0x00007ff0ba9af000)
	libm.so.6 => /lib64/libm.so.6 (0x00007ff0ba8d4000)
	libgcc_s.so.1 => /lib64/libgcc_s.so.1 (0x00007ff0ba8b9000)
	libc.so.6 => /lib64/libc.so.6 (0x00007ff0ba6c6000)
	/lib64/ld-linux-x86-64.so.2 (0x00007ff0babf0000)

Fedora 34 / glibc-2.33:

[kev@f34-1 gdb]$ ldd testsuite/outputs/gdb.threads/tls/tls
	linux-vdso.so.1 (0x00007fff32dc0000)
	libpthread.so.0 => /lib64/libpthread.so.0 (0x00007f815f6de000)
	libstdc++.so.6 => /lib64/libstdc++.so.6 (0x00007f815f4bf000)
	libm.so.6 => /lib64/libm.so.6 (0x00007f815f37b000)
	libgcc_s.so.1 => /lib64/libgcc_s.so.1 (0x00007f815f360000)
	libc.so.6 => /lib64/libc.so.6 (0x00007f815f191000)
	/lib64/ld-linux-x86-64.so.2 (0x00007f815f721000)

Note that libpthread is missing from the ldd output for the
glibc-2.33.9000 machine.

This means that (unless we happen to think of some entirely different
mechanism), we'll now need to potentially match "libc" in addition to
"libpthread" as libraries which might be thread libraries.  This
accounts for the change made in solib.c.  Note that the new code
attempts to match "/libc." via strstr().  That trailing dot (".")
avoids inadvertently matching libraries such as libcrypt (and
all the other many libraries which begin with "libc").

To avoid attempts to load libthread_db when encountering older
versions of libc, we now attempt to find "pthread_create" (which is a
symbol that we'd expect to be in any pthread library) in the
associated objfile.  This accounts for the changes in
linux-thread-db.c.

I think that other small adjustments will need to be made elsewhere
too.  I've been working through regressions on my glibc-2.33.9000
machine; I've fixed some fairly "obvious" changes in the testsuite
(which are in other commits).  For the rest, it's not yet clear to me
whether the handful of remaining failures represent a problem in glibc
or gdb.  I'm still investigating, however, I'll note that these are
problems that I only see on my glibc-2.33.9000 machine.

gdb/ChangeLog:

	* solib.c (libpthread_name_p): Match "libc" in addition
	to "libpthread".
	* linux-thread-db.c (libpthread_objfile_p): New function.
	(libpthread_name_p): Adjust preexisting callers to use
	libpthread_objfile_p().