fix: warning 'noreturn' function does return on ppc

fix: warning 'noreturn' function does return on ppc

On a ppc64 system with gcc 9.5.0 I get the following error when building
with -O0 :

/usr/include/urcu/uatomic/generic.h: In function 'void _uatomic_link_error()':
/usr/include/urcu/uatomic/generic.h:53:1: warning: 'noreturn' function does return
   53 | }
      | ^

Split the inline function in 2 variants and apply the noreturn attribute
only on the builtin_trap one.

Change-Id: I5ae8e764c4cc27af0463924a653b9eaa9f698c34
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>