[gdb/testsuite] Fix language in dw2-bad-mips-linkage-name.exp

[gdb/testsuite] Fix language in dw2-bad-mips-linkage-name.exp

The test-case gdb.dwarf2/dw2-bad-mips-linkage-name.exp has a CU with
language C, which contains a subprogram with a C++-mangled name as its
DW_AT_mips_linkage_name attribute.

Fix this by changing the language of the CU to C++.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-04-24  Tom de Vries  <tdevries@suse.de>

	* gdb.dwarf2/dw2-bad-mips-linkage-name.exp: Set language of CU to
	C++.