uma: increase alignment to 128 bytes on amd64

Current UMA internals are not suited for efficient operation in
multi-socket environments. In particular there is very common use of
MAXCPU arrays and other fields which are not always properly aligned and
are not local for target threads (apart from the first node of course).
Turns out the existing UMA_ALIGN macro can be used to mostly work around
the problem until the code get fixed. The current setting of 64 bytes
runs into trouble when adjacent cache line prefetcher gets to work.

An example 128-way benchmark doing a lot of malloc/frees has the following
instruction samples:

before:
kernel`lf_advlockasync+0x43b            32940
          kernel`malloc+0xe5            42380
           kernel`bzero+0x19            47798
   kernel`spinlock_exit+0x26            60423
         kernel`0xffffffff80            78238
                         0x0           136947
   kernel`uma_zfree_arg+0x46           159594
 kernel`uma_zalloc_arg+0x672           180556
   kernel`uma_zfree_arg+0x2a           459923
 kernel`uma_zalloc_arg+0x5ec           489910

after:
            kernel`bzero+0xd            46115
kernel`lf_advlockasync+0x25f            46134
kernel`lf_advlockasync+0x38a            49078
   kernel`fget_unlocked+0xd1            49942
kernel`lf_advlockasync+0x43b            55392
          kernel`copyin+0x4a            56963
           kernel`bzero+0x19            81983
   kernel`spinlock_exit+0x26            91889
         kernel`0xffffffff80           136357
                         0x0           239424

See the review for more details.

Reviewed by:	kib
Differential Revision:	https://reviews.freebsd.org/D15346

sys/vm/uma_int.h

@@ -177,7 +177,7 @@ struct uma_hash {
  * align field or structure to cache line
  */
 #if defined(__amd64__)
-#define UMA_ALIGN	__aligned(CACHE_LINE_SIZE)
+#define UMA_ALIGN	__aligned(128)
 #else
 #define UMA_ALIGN
 #endif