[gdb/testsuite] Handle no glibc debuginfo in gdb.base/solib-corrupted.exp

[gdb/testsuite] Handle no glibc debuginfo in gdb.base/solib-corrupted.exp

When running test-case gdb.base/solib-corrupted.exp on SLE-11, I get:
...
(gdb) PASS: gdb.base/solib-corrupted.exp: normal list
p/x _r_debug->r_map->l_next = _r_debug->r_map^M
'_r_debug' has unknown type; cast it to its declared type^M
(gdb) FAIL: gdb.base/solib-corrupted.exp: make solibs looping
...

The reason that _r_debug has unknown type is that glibc debuginfo is not
installed.  The test-case attempts to detect this but doesn't handle this
particular error string.

Fix this by adding the "unknown type" line to the regexp detecting missing
glibc debuginfo.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-12-14  Tom de Vries  <tdevries@suse.de>

	PR testsuite/26962
	* gdb.base/solib-corrupted.exp: Handle "'_r_debug' has unknown type;
	cast it to its declared type".