lttng-tools: fix compile error for x32

lttng-tools: fix compile error for x32

Fix build error of src/common/utils.c for x32:

| .../src/common/utils.c: Assembler messages:
| .../src/common/utils.c:1026: Error: register type mismatch for `bsr'
| .../src/common/utils.c:1028: Error: operand type mismatch for `movq'
| make[3]: *** [utils.lo] Error 1

[YOCTO #13081]

Signed-off-by: Kai Kang <kai.kang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>