libsel4muslcsys: Add some missing thread routines

A C library may call set_thread_area on startup and tkill on shutdown,
so we need to implement stubs of these

libsel4muslcsys/src/sys_exit.c

diff --git a/libsel4muslcsys/src/sys_exit.c b/libsel4muslcsys/src/sys_exit.c
index d2e1ec8..3872fab 100644
--- a/libsel4muslcsys/src/sys_exit.c
+++ b/libsel4muslcsys/src/sys_exit.c
@@ -61,6 +61,14 @@
 }
 
 long
+sys_tkill(va_list ap)
+{
+    printf("%s assuming self kill\n", __FUNCTION__);
+    sel4_abort();
+    return 0;
+}
+
+long
 sys_exit_group(va_list ap)
 {
     printf("Ignoring call to %s\n", __FUNCTION__);

libsel4muslcsys/src/sys_thread.c

diff --git a/libsel4muslcsys/src/sys_thread.c b/libsel4muslcsys/src/sys_thread.c
index 6bea175..84704d4 100644
--- a/libsel4muslcsys/src/sys_thread.c
+++ b/libsel4muslcsys/src/sys_thread.c
@@ -12,7 +12,12 @@
 #include <bits/errno.h>
 
 long sys_set_thread_area(va_list ap) {
-    return -ENOSYS;
+    /* As part of the initialization of the C library we need to set the
+     * thread area (also knows as the TLS base) for thread local storage.
+     * As we do not properly support TLS we just ignore this call. Will
+     * be fine provided we do not create multiple threads (through libc)
+     * or use TLS */
+    return 0;
 }
 
 long sys_set_tid_address(va_list ap) {

libsel4muslcsys/src/syscalls.h

diff --git a/libsel4muslcsys/src/syscalls.h b/libsel4muslcsys/src/syscalls.h
index fb5dba3..757cb39 100644
--- a/libsel4muslcsys/src/syscalls.h
+++ b/libsel4muslcsys/src/syscalls.h
@@ -22,6 +22,7 @@
 long sys_gettid(va_list ap);
 long sys_getpid(va_list ap);
 long sys_tgkill(va_list ap);
+long sys_tkill(va_list ap);
 long sys_exit_group(va_list ap);
 long sys_open(va_list ap);
 long sys_close(va_list ap);

libsel4muslcsys/src/vsyscall.c

diff --git a/libsel4muslcsys/src/vsyscall.c b/libsel4muslcsys/src/vsyscall.c
index ed1ece2..45c4561 100644
--- a/libsel4muslcsys/src/vsyscall.c
+++ b/libsel4muslcsys/src/vsyscall.c
@@ -30,6 +30,7 @@
     [__NR_gettid] = sys_gettid,
     [__NR_getpid] = sys_getpid,
     [__NR_tgkill] = sys_tgkill,
+    [__NR_tkill] = sys_tkill,
     [__NR_exit_group] = sys_exit_group,
     [__NR_open] = sys_open,
     [__NR_close] = sys_close,