LoongArch: Adjust process management for 32BIT/64BIT

LoongArch: Adjust process management for 32BIT/64BIT

Adjust process management for both 32BIT and 64BIT, including: CPU
context switching, FPU loading/restoring, process dumping and process
tracing routines.

Q: Why modify switch.S?
A: LoongArch32 has no ldptr.d/stptr.d instructions, and asm offsets of
   thead_struct members are too large to be filled in the 12b immediate
   field of ld.w/st.w.

Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>