Fix: compile fails for x32 arch

LTTng-Tools fails to compile for the x32 ABI:

| .../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'

Add a macro to only use the x86_64 inline assembly version of fls_u64()
when the LP64 ABI is used.

Signed-off-by: Kai Kang <kai.kang@windriver.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>