[musl] loongarch64 atomics not working?

[Rich Felker <dalias@libc.org>]

There's been a report of mksh hanging on loongarch64, at least under
qemu, apparently hanging in a_cas_p:

(gdb) run
Starting program: /mksh 
^C
Program received signal SIGINT, Interrupt.
a_cas_p (p=0x120054288 <vdso_func>, t=0x12003b970 <cgt_init>, s=0x7fffffffc34c)
    at ./src/internal/atomic.h:94
warning: 94     ./src/internal/atomic.h: No such file or directory
(gdb) bt
#0  a_cas_p (p=0x120054288 <vdso_func>, t=0x12003b970 <cgt_init>, 
    s=0x7fffffffc34c) at ./src/internal/atomic.h:94
#1  cgt_init (clk=0, ts=0x7ffffffefb60) at src/time/clock_gettime.c:51
#2  0x000000012003ba4c in __clock_gettime (clk=clk@entry=0, 
    ts=ts@entry=0x7ffffffefb60) at src/time/clock_gettime.c:67
#3  0x000000012003830c in gettimeofday (tv=tv@entry=0x7ffffffefba0, 
    tz=tz@entry=0x0) at src/time/gettimeofday.c:9
#4  0x000000012002f098 in change_winsz () at var.c:1718
#5  0x0000000120000348 in main_init (lp=<synthetic pointer>, 
    sp=<synthetic pointer>, argv=0x7ffffffefd18, argc=1) at main.c:369
#6  main (argc=<optimized out>, argv=<optimized out>) at main.c:738

This is very basic usage, just the vdso clock_gettime init code trying
to replace a pointer atomically. Is it working on real hardware? I'm
trying to figure out if this is a qemu bug, or if the asm or the asm
argument constraints are wrong in musl's
arch/loongarch64/atomic_arch.h.

Rich