linux-user: substantial memory leak when threads are created and destroyed
Description of problem:
Substantial memory leak when the following simple program is executed on `qemu-arm`,
```c
// compile with `arm-none-linux-gnueabihf-gcc test_qemu.c -o test_qemu.out -pthread`

#include <assert.h>
#include <pthread.h>

#define MAGIC_RETURN ((void *)42)

void *thread_main(void *arg)
{
    return MAGIC_RETURN;
}

int main(int argc, char *argv[])
{
    size_t i;
    for (i = 0;; i++)
    {
        pthread_t thread;
        assert(pthread_create(&thread, NULL, thread_main, NULL) == 0);
        void *ret;
        assert(pthread_join(thread, &ret) == 0);
        assert(ret == MAGIC_RETURN);
    }

    return 0;
}
```
Steps to reproduce:
1. 
```
export TOOLCHAIN_PREFIX=arm-none-linux-gnueabihf
export ARMSDK=/${TOOLCHAIN_PREFIX}
export SYSROOT=${ARMSDK}/${TOOLCHAIN_PREFIX}/libc
export CC=${ARMSDK}/bin/${TOOLCHAIN_PREFIX}-gcc
```
2. Download the arm toolchain: `curl --output ${TOOLCHAIN_PREFIX}.tar.xz -L 'https://developer.arm.com/-/media/Files/downloads/gnu-a/10.2-2020.11/binrel/gcc-arm-10.2-2020.11-x86_64-arm-none-linux-gnueabihf.tar.xz?revision=d0b90559-3960-4e4b-9297-7ddbc3e52783&la=en&hash=985078B758BC782BC338DB947347107FBCF8EF6B'`
3. `mkdir -p ${ARMSDK} && tar xf ${TOOLCHAIN_PREFIX}.tar.xz -C ${ARMSDK} --strip-components=1`
4. `$CC test_qemu.c -o test_qemu.out -pthread`
5. `qemu-arm -L $SYSROOT ./test_qemu.out`
6. Observe memory usage keeps ramping up and crashes the process once out of memory.
Additional information:
Valgrind annotation logs [annot.log](/uploads/f8d05d8f216d5a589e8da0758a345de6/annot.log) generated by a local build on master@0a301624c2f4ced3331ffd5bce85b4274fe132af from
```bash
valgrind --xtree-memory=full --xtree-memory-file=xtmemory.kcg bin/debug/native/qemu-arm -L $SYSROOT /mnt/f/test_qemu3.out
# Send CTRL-C before the process crashes due to oom
callgrind_annotate --auto=yes --inclusive=yes --sort=curB:100,curBk:100,totB:100,totBk:100,totFdB:100,totFdBk:100  xtmemory.kcg > annot.log
```

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