[gdb/symtab] Make per_cu->m_lang atomic

[gdb/symtab] Make per_cu->m_lang atomic

When building gdb with -fsanitize=thread and running test-case
gdb.dwarf2/inlined_subroutine-inheritance.exp, we run into a data race
between:
...
  Read of size 1 at 0x7b2000003010 by thread T4:
    #0 packed<language, 1ul>::operator language() const packed.h:54
    #1 dwarf2_per_cu_data::set_lang(language) read.h:363
...
and:
...
  Previous write of size 1 at 0x7b2000003010 by main thread:
    #0 dwarf2_per_cu_data::set_lang(language) read.h:365
...

Fix this by making per_cu->m_lang atomic.

Tested on x86_64-linux.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29286