[gdb/testsuite] Handle asm frame in gdb.python/py-missing-objfile.exp

[gdb/testsuite] Handle asm frame in gdb.python/py-missing-objfile.exp

On arm-linux, with test-case gdb.python/py-missing-objfile.exp I get:
...
(gdb) whatis global_exec_var^M
type = volatile exec_type^M
(gdb) FAIL: $exp: initial sanity check: whatis global_exec_var
...
instead of the expected "type = volatile struct exec_type".

The problem is that the current language is ASM instead of C, because the
inner frame at the point of the core dump has language ASM:
...
 #0  __libc_do_syscall () at libc-do-syscall.S:47
 #1  0xf7882920 in __pthread_kill_implementation () at pthread_kill.c:43
 #2  0xf784df22 in __GI_raise (sig=sig@entry=6) at raise.c:26
 #3  0xf783f03e in __GI_abort () at abort.c:73
 #4  0x009b0538 in dump_core () at py-missing-objfile.c:34
 #5  0x009b0598 in main () at py-missing-objfile.c:46
...

Fix this by manually setting the language to C.

Tested on arm-linux and x86_64-linux.

PR testsuite/32445
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=32445