FROMLIST: arm64: fix alternatives with LLVM's integrated assembler

LLVM's integrated assembler fails with the following error when
building KVM:

  <inline asm>:12:6: error: expected absolute expression
   .if kvm_update_va_mask == 0
       ^
  <inline asm>:21:6: error: expected absolute expression
   .if kvm_update_va_mask == 0
       ^
  <inline asm>:24:2: error: unrecognized instruction mnemonic
          NOT_AN_INSTRUCTION
          ^
  LLVM ERROR: Error parsing inline asm

These errors come from ALTERNATIVE_CB and __ALTERNATIVE_CFG,
which test for the existence of the callback parameter in inline
assembly using the following expression:

  " .if " __stringify(cb) " == 0\n"

This works with GNU as, but isn't supported by LLVM. This change
splits __ALTERNATIVE_CFG and ALTINSTR_ENTRY into separate macros
to fix the LLVM build.

Bug: 145210207
Change-Id: I3f80fca8aafdac4e185f79ce5a4eee9ba367bb33
(am from https://lore.kernel.org/patchwork/patch/1146950/)
Link: https://github.com/ClangBuiltLinux/linux/issues/472


Signed-off-by: Sami Tolvanen <samitolvanen@google.com>