libthr: add LoongArch support

[zhaoxiaoqiang007_gmail.com]

Summary

Add pthread_md.h TLS/TP glue and the libthread_db register mapping.

Signed-off-by: Xiaoqiang Zhao <[e-mail]>

[shipujin.t_gmail.com]

lib/libthr/arch/loongarch/include/pthread_md.h

38

CPU_SPINWAIT is used in spinlock loops to give the CPU a lightweight hint (such as x86's pause) to reduce power consumption and memory bus contention, but LoongArch has no equivalent instruction. Therefore, like RISC-V, AArch64, ARM, and PowerPC, this should define the macro as #define CPU_SPINWAIT rather than using a compiler memory barrier.

Suggested change to:

#define	CPU_SPINWAIT