elf: Introduce <dl-call_tls_init_tp.h> and call_tls_init_tp (bug 29249)

This makes it more likely that the compiler can compute the strlen
argument in _startup_fatal at compile time, which is required to
avoid a dependency on strlen this early during process startup.

Reviewed-by: Szabolcs Nagy <szabolcs.nagy@arm.com>

sysdeps/unix/sysv/linux/arm/tls.h

@@ -33,8 +33,7 @@
 # define TLS_INIT_TP(tcbp) \
   ({ long int result_var;						\
      result_var = INTERNAL_SYSCALL_CALL (set_tls, (tcbp));		\
-     INTERNAL_SYSCALL_ERROR_P (result_var)				\
-       ? "unknown error" : NULL; })
+     !INTERNAL_SYSCALL_ERROR_P (result_var); })
 
 #endif /* __ASSEMBLER__ */