Add AArch64 as an experimental target.

This patch adds support for AArch64 (ARM's 64-bit architecture) to
LLVM in the "experimental" category. Currently, it won't be built
unless requested explicitly.

This initial commit should have support for:
    + Assembly of all scalar (i.e. non-NEON, non-Crypto) instructions
      (except the late addition CRC instructions).
    + CodeGen features required for C++03 and C99.
    + Compilation for the "small" memory model: code+static data <
      4GB.
    + Absolute and position-independent code.
    + GNU-style (i.e. "__thread") TLS.
    + Debugging information.

The principal omission, currently, is performance tuning.

This patch excludes the NEON support also reviewed due to an outbreak of
batshit insanity in our legal department. That will be committed soon bringing
the changes to precisely what has been approved.

Further reviews would be gratefully received.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@174054 91177308-0d34-0410-b5e6-96231b3b80d8

test/CodeGen/AArch64/zero-reg.ll

@@ -0,0 +1,31 @@
+; RUN: llc -verify-machineinstrs < %s -march=aarch64 | FileCheck %s
+
+@var32 = global i32 0
+@var64 = global i64 0
+
+define void @test_zr() {
+; CHECK: test_zr:
+
+  store i32 0, i32* @var32
+; CHECK: str wzr, [{{x[0-9]+}}, #:lo12:var32]
+  store i64 0, i64* @var64
+; CHECK: str xzr, [{{x[0-9]+}}, #:lo12:var64]
+
+  ret void
+; CHECK: ret
+}
+
+define void @test_sp(i32 %val) {
+; CHECK: test_sp:
+
+; Important correctness point here is that LLVM doesn't try to use xzr
+; as an addressing register: "str w0, [xzr]" is not a valid A64
+; instruction (0b11111 in the Rn field would mean "sp").
+  %addr = getelementptr i32* null, i64 0
+  store i32 %val, i32* %addr
+; CHECK: mov x[[NULL:[0-9]+]], xzr
+; CHECK: str {{w[0-9]+}}, [x[[NULL]]]
+
+  ret void
+; CHECK: ret
+}