fix alignment for atomics on PPC

Following the 64-bit PowerPC ELF ABI specification all types should be
aligned to their own size.

base/atomics.jl

@@ -207,7 +207,12 @@ function alignment{T}(::Type{T})
     # types, with the following exceptions: On 32- and 64-bit x86 platforms
     # and on 64-bit SPARC platforms, atomic types of size 1, 2, 4, 8 or 16 -byte
     # have the alignment that matches the size.
-    if Sys.ARCH === :x86_64 || Sys.ARCH === :i686
+    #
+    # Note for PPC:
+    # The OpenPower ABI for Linux Supplement -- 64-Bit ELF V2 ABI Specification
+    # defines that the alignments of types is sizeof(T).
+    if Sys.ARCH === :x86_64 || Sys.ARCH === :i686 ||
+       Sys.ARCH === :ppc64le || Sys.ARCH === Sys.ARCH === :powerpc64le
         return sizeof(T)
     end
     return WORD_SIZE ÷ 8

test/threads.jl

@@ -273,7 +273,8 @@ let atomic_types = [Int8, Int16, Int32, Int64, Int128,
                     Float16, Float32, Float64]
     # Temporarily omit 128-bit types on 32bit x86
     # 128-bit atomics do not exist on AArch32.
-    # And we don't support them yet on power.
+    # And we don't support them yet on power, because they are lowered
+    # to `__sync_lock_test_and_set_16`.
     if Sys.ARCH === :i686 || startswith(string(Sys.ARCH), "arm") ||
        Sys.ARCH === :powerpc64le || Sys.ARCH === :ppc64le
         filter!(T -> sizeof(T)<=8, atomic_types)