usdt: fix bpftrace parse error on arm64

bpftrace reports parse error on arm64:
"parse error:
8@x0 4@[x21, #:lo12:.LANCHOR2]
"
using "nr" to override the default constraints string can fix this.

//the default is:
 stapsdt              0x0000004b       NT_STAPSDT (SystemTap probe descriptors)
    Provider: spdk
    Name: nvmf_tgt_state
    Location: 0x000000000006e664, Base: 0x00000000001ca6f0, Semaphore: 0x0000000000000000
    Arguments: 8@x0 4@[x21, #:lo12:.LANCHOR2]

//using "nr" as the constraints string:
 stapsdt              0x00000036       NT_STAPSDT (SystemTap probe descriptors)
    Provider: spdk
    Name: nvmf_tgt_state
    Location: 0x000000000006f680, Base: 0x00000000001cb730, Semaphore: 0x0000000000000000
    Arguments: 8@x0 4@x1

Change-Id: I25736dcf5a8593d2c156da41a2e28532ab8fa570
Signed-off-by: Richael Zhuang <richael.zhuang@arm.com>
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/14642


Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Community-CI: Mellanox Build Bot
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Shuhei Matsumoto <smatsumoto@nvidia.com>