gdb/testsuite: make 'c' default language for get/test compiler info

gdb/testsuite: make 'c' default language for get/test compiler info

This commit is a minor cleanup for the two functions (in gdb.exp)
get_compiler_info and test_compiler_info.

Instead of using the empty string as the default language, and just
"knowing" that this means the C language.  Make this explicit.  The
language argument now defaults to "c" if not specified, and the if
chain in get_compiler_info that checks the language not explicitly
handles "c" and gives an error for unknown languages.

This is a good thing, now that the API appears to take a language, if
somebody does:

  test_compiler_info "xxxx" "rust"

to check the version of the rust compiler then we will now give an
error rather than just using the C compiler and leaving the user
having to figure out why they are not getting the results they
expect.

After a little grepping, I think the only place we were explicitly
passing the empty string to either get_compiler_info or
test_compiler_info was in gdb_compile_shlib_1, this is now changed to
pass "c" as the default language.

There should be no changes to the test results after this commit.