Fix race in DWARF reader

Fix race in DWARF reader

The recent change to record the DWARF language in the per-CU data
yielded a race warning in my testing:

ThreadSanitizer: data race ../../binutils-gdb/gdb/dwarf2/read.c:21779 in prepare_one_comp_unit

This patch fixes the bug by applying the same style of fix that was
done for the ordinary (gdb) language.

I wonder if this code could be improved.  Requiring an atomic for the
language in particular seems unfortunate, as it is often consulted
during index finalization.  However, I haven't investigated this.

Regression tested on x86-64 Fedora 38.

Reviewed-by: Tom de Vries <tdevries@suse.de>