powerpc/nohash: Fix build with llvm-as

When using the LLVM integrated assembler (llvm-as), the book3e build
fails with:

  arch/powerpc/mm/nohash/tlb_low_64e.S:354:2: error: invalid instruction
   tlbilxva 0,%r15
   ^

tlbilxva is an extended mnemonic for tlbilx, but llvm-as also doesn't
support tlbilx, despite it being an e500mc instruction.

Fix it by using the existing PPC_TLBILX_VA macro. The resulting binary
is identical when building with binutils.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20230216112915.1681631-1-mpe@ellerman.id.au