Build system changes to enable MCJIT on AArch64

These changes just allow AArch64 to take part in the MCJIT world when
built correctly.
AArch64: implement first relocation required for MCJIT

R_AARCH64_PCREL32 is present in even trivial .eh_frame sections and so
is required to compile any function without the "nounwind" attribute.

This change implements very basic infrastructure in the RuntimeDyldELF
file and allows (for example) the test-shift.ll MCJIT test to pass
on AArch64.
AArch64: implement relocations for global access

The large memory model (default and main viable for JIT) emits
addresses in need of relocation as
    movz x0, #:abs_g3:somewhere
    movk x0, #:abs_g2_nc:somewhere
    movk x0, #:abs_g1_nc:somewhere
    movk x0, #:abs_g0_nc:somewhere

To support this we must implement those four relocations in the
dynamic loader.

This allows (for example) the test-global.ll MCJIT test to pass on
AArch64.
AArch64: add stubs to support long function calls on MCJIT

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.
AArch64: implement 64-bit absolute relocation in MCJIT

This is about the simplest relocation, but surprisingly rare in actual
code.

It occurs in (for example) the MCJIT test test-ptr-reloc.ll.
AArch64: enable MCJIT and tests now that everything passes.

This removes dire warnings about AArch64 being unsupported and enables
the tests when appropriate on this platform.