LoongArch/ftrace: Add recordmcount support

Recordmcount utility under scripts is run, after compiling each object,
to find out all the locations of calling _mcount() and put them into
specific seciton named __mcount_loc.

Then the linker collects all such information into a table in the kernel
image (between __start_mcount_loc and __stop_mcount_loc) for later use
by ftrace.

This patch adds LoongArch specific definitions to identify such locations.
And on LoongArch, only the C version is used to build the kernel now that
CONFIG_HAVE_C_RECORDMCOUNT is on.

Acked-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Qing Zhang <zhangqing@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>