[AArch64] Correct inline assembly test case for S modifier [NFC]

The existing test for the AArch64 inline assembly constraint S uses the
A and L modifiers. These modifiers were implemented in the original
AArch64 backend but were not carried forward to the merged backend. The
A is associated with ADRP and does nothing, the L is associated with
:lo12: . Given that A and L are not supported by GCC and not supported
by the new implementation of constraint S in LLVM (see D46745) I've
altered the test to put :lo12: directly in the string so that A and L
are not needed.

Differential Revision: https://reviews.llvm.org/D46932

llvm-svn: 332606