Add support for gprofng on LoongArch

[mengqinggang]

Hi,

I am porting gprofng to LoongArch architecture.
I have some question about stack_unwind function in libcollector/unwind.c.
Is it possible to use .eh_frame section for stack unwinding?
Aarch64 stack unwind is simple because x29, x30 register saved on SP and 
(SP + 8).
But x86_64 stack unwind need to walk through instructions, it looks very 
complicated.
Is there any other method for stack unwind?

Thank you.