gdb: LoongArch: Implement loongarch_scan_prologue()

If can't determine prologue from the symbol table, need to examine
instructions. Implement loongarch_scan_prologue() to analyze the
function prologue from START_PC to LIMIT_PC, return the address of
the first instruction past the prologue.

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