[gdb/testsuite] Fix c-linkage-name.exp with {cc-with-gdb-index,readnow}.exp

When running test-case gdb.base/c-linkage-name.exp with target board
cc-with-gdb-index.exp, I see:
...
FAIL: gdb.base/c-linkage-name.exp: maint info psymtab: c-linkage-name-2.c: no
FAIL: gdb.base/c-linkage-name.exp: maint info psymtab: c-linkage-name-2.c: yes
...
The FAILs are due to the fact that partial symbol tables are not generated for
indexed executables.

When running the same test-case with target board readnow.exp, I see:
...
FAIL: gdb.base/c-linkage-name.exp: maint info psymtab: c-linkage-name-2.c: no
FAIL: gdb.base/c-linkage-name.exp: print symada__cS before partial symtab \
  expansion
FAIL: gdb.base/c-linkage-name.exp: maint info psymtab: c-linkage-name-2.c: yes
...
The "maint info psymtab" FAILs are also due to fact that the partial symbol
tables not generated, but in this case it's because the symtabs are fully
expanded upon load due to using -readnow.  The "print symada__cS before
partial symtab expansion" test intends to test the state before symbol table
expansion, and with -readnow that's not possible.

Mark these FAILs as UNSUPPORTED.

Tested on x86_64-linux, with native, and target boards cc-with-gdb-index.exp,
cc-with-debug-names.exp and readnow.exp.

gdb/testsuite/ChangeLog:

2020-03-30  Tom de Vries  <tdevries@suse.de>

	* gdb.base/c-linkage-name.exp: Use readnow call to mark a test
	unsupported.
	(verify_psymtab_expanded): Move ...
	* lib/gdb.exp (verify_psymtab_expanded): ... here.  Add unsupported
	test.
	(readnow): New proc.