[gdb/testsuite] Fix c-linkage-name.exp with -flto

[gdb/testsuite] Fix c-linkage-name.exp with -flto

When running test-case gdb.base/c-linkage-name.exp with target board
unix/-flto/-O0/-flto-partition=none/-ffat-lto-objects, I run into:
...
PASS: 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
...

The test-case tries to print a symbol before and after symtab expansion.

And it tries to ensure (since commit 13c3a74afb) that the symtab containing
the symbol is not yet expanded when doing the 'before' print, by placing the
symbol in a different CU (c-linkage-name-2.c) from the one containing main
(c-linkage-name.c), such that when we load the exec and expand the symtab
containing main, the symtab containing the symbol isn't.

The generated debug info for the test-case when using mentioned target board
however is structured like this:
...
 <0><d2>: Abbrev Number: 1 (DW_TAG_compile_unit)
    <d8>   DW_AT_name        : <artificial>
 <1><f4>: Abbrev Number: 2 (DW_TAG_imported_unit)
    <f5>   DW_AT_import      : <0x16b>  [Abbrev Number: 1]
 <1><f9>: Abbrev Number: 2 (DW_TAG_imported_unit)
    <fa>   DW_AT_import      : <0x19c>  [Abbrev Number: 1]
 <1><fe>: Abbrev Number: 3 (DW_TAG_subprogram)
    <ff>   DW_AT_abstract_origin: <0x17d>
 <1><115>: Abbrev Number: 4 (DW_TAG_variable)
    <116>   DW_AT_abstract_origin: <0x1ce>
 <0><16b>: Abbrev Number: 1 (DW_TAG_compile_unit)
    <171>   DW_AT_name        : c-linkage-name.c
 <1><17d>: Abbrev Number: 2 (DW_TAG_subprogram)
    <17e>   DW_AT_name        : main
 <0><19c>: Abbrev Number: 1 (DW_TAG_compile_unit)
    <1a2>   DW_AT_name        : c-linkage-name-2.c
 <1><1ce>: Abbrev Number: 5 (DW_TAG_variable)
    <1cf>   DW_AT_name        : mundane
    <1d6>   DW_AT_linkage_name: symada__cS
...

So, the CU named <artificial> contains both the concrete main and the concrete
symbol, which explains the FAIL.

The first test should fail, but passes for two reasons.

First of all, due to PR symtab/25700, we have two regular partial symtabs
c-linkage-name-2.c instead of one, and one of them is expanded, the other one
not:
...
  { psymtab c-linkage-name-2.c ((struct partial_symtab *) 0x38d6f60)
    readin yes
  { psymtab c-linkage-name-2.c ((struct partial_symtab *) 0x38d6fe0)
    readin no
...

And then there's the include symtab, which is also not expanded:
...
  { psymtab c-linkage-name-2.c ((struct partial_symtab *) 0x38143e0)
    readin no
...

Fix the FAIL by explicitly setting the language before load, changing the
language setting from auto/c to manual/c, such that the symtab containing main
is no longer expanded.

And make the symtab expansion testing more robust by using the output of
"maint info symtabs" instead of "maint info psymtabs".

Tested on x86_64-linux, using native and target boards cc-with-gdb-index.exp,
cc-with-debug-names.exp, readnow.exp and
unix/-flto/-O0/-flto-partition=none/-ffat-lto-objects.

gdb/testsuite/ChangeLog:

2020-03-31  Tom de Vries  <tdevries@suse.de>

	* gdb.base/c-linkage-name.exp: Fix test-case comment.  Set language to
	c.  Use "maint info symtabs" to check symtab expansion.