s390x: fix inline asm on gcc10

Fix compilation issues on 390x with gcc 10.

Simply mark the inline functions that lead to a .insn with a variable
opcode as __always_inline, to make gcc 10 happy.

Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Janosch Frank <frankja@linux.ibm.com>
Message-Id: <20200727102643.15439-1-imbrenda@linux.ibm.com>
Signed-off-by: Janosch Frank <frankja@linux.ibm.com>