LoongArch: Add support to annotate tablejump

This is per the request from the kernel developers.  For generating the
ORC unwind info, the objtool program needs to analysis the control flow
of a .o file.  If a jump table is used, objtool has to correlate the
jump instruction with the table.

On x86 (where objtool was initially developed) it's simple: a relocation
entry natrually correlates them because one single instruction is used
for table-based jump.  But on an RISC machine objtool would have to
reconstruct the data flow if it must find out the correlation on its
own.

So, emit an additional section to store the correlation info as pairs of
addresses, each pair contains the address of a jump instruction (jr) and
the address of the jump table.  This is very trivial to implement in
GCC.

gcc/ChangeLog:

	* config/loongarch/genopts/loongarch.opt.in
	(mannotate-tablejump): New option.
	* config/loongarch/loongarch.opt: Regenerate.
	* config/loongarch/loongarch.md (tablejump<mode>): Emit
	additional correlation info between the jump instruction and the
	jump table, if -mannotate-tablejump.
	* doc/invoke.texi: Document -mannotate-tablejump.

gcc/testsuite/ChangeLog:

	* gcc.target/loongarch/jump-table-annotate.c: New test.

Suggested-by: Tiezhu Yang <yangtiezhu@loongson.cn>

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