[gdb/breakpoints] Fix longjmp master breakpoint with separate debug info

When running test-case gdb.base/longjmp.exp with target board unix/-m32, we
run into:
...
(gdb) next^M
Warning:^M
Cannot insert breakpoint 0.^M
Cannot access memory at address 0x7dbf7353^M
^M
__libc_siglongjmp (env=0x804a040 <env>, val=1) at longjmp.c:28^M
28        longjmps++;^M
(gdb) FAIL: gdb.base/longjmp.exp: next over longjmp(1)
...

The failure to access memory happens in i386_get_longjmp_target and is due to
glibc having pointer encryption (aka "pointer mangling" or "pointer guard") of
the long jump buffer.  This is a known problem.

In create_longjmp_master_breakpoint (which attempts to install a master
longjmp breakpoint) a preference scheme is present, which installs a
probe breakpoint if a libc:longjmp probe is present, and otherwise falls back
to setting breakpoints at the names in the longjmp_names array.

But in fact, both the probe breakpoint and the longjmp_names breakpoints are
set.  The latter ones are set when processing libc.so.debug, and the former
one when processing libc.so.  In other words, this is the longjmp variant of
PR26881, which describes the same problem for master exception breakpoints.

This problem only triggers when the glibc debug info package is installed,
which is not due to the debug info itself in libc.so.debug, but due to the
minimal symbols (because create_longjmp_master_breakpoint uses minimal symbols
to translate the longjmp_names to addresses).

The problem doesn't trigger for -m64, because there tdep->jb_pc_offset is not
set.

Fix this similar to commit 1940319c (the fix for PR26881): only install
longjmp_names breakpoints in libc.so/libc.so.debug if installing the
libc:longjmp probe in libc.so failed.

Tested on x86_64-linux.

gdb/ChangeLog:

2021-01-28  Tom de Vries  <tdevries@suse.de>

	PR breakpoints/27205
	* breakpoint.c (create_longjmp_master_breakpoint_probe)
	(create_longjmp_master_breakpoint_names): New function, factored out
	of ...
	(create_longjmp_master_breakpoint): ... here.  Only try to install
	longjmp_names breakpoints in libc.so/libc.so.debug if installing probe
	breakpoint in libc.so failed.