gdb: LoongArch: Update status of the entire regset in regcache

In the current code, when a register is fetched, the entire regset
are fetched via ptrace, but only this register status is updated in
regcache, it needs to fetch the same regset through ptrace again if
another register in this regset is fetched later, this is obviously
unnecessary. It is proper to update the status of the entire regset
in regcache when fetching a register via ptrace.

Signed-off-by: Hui Li <lihui@loongson.cn>
Reviewed-By: Tom Tromey <tom@tromey.com>
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>