As with global accesses, external functions could exist anywhere in
memory. Therefore the stub must create a complete 64-bit address. This
patch implements the fragment as (roughly):
    movz x16, #:abs_g3:somefunc
    movk x16, #:abs_g2_nc:somefunc
    movk x16, #:abs_g1_nc:somefunc
    movk x16, #:abs_g0_nc:somefunc
    br x16

In principle we could save 4 bytes by using a literal-load instead,
but it is unclear that would be more efficient and can only be tested
when real hardware is readily available.

This allows (for example) the MCJIT test 2003-05-07-ArgumentTest to
pass on AArch64.

llvm-svn: 181133