libxl/arm: Fix build on arm64 + acpi

libxl/arm: Fix build on arm64 + acpi

With musl, the build fails with the following errors:

  actypes.h:202:2: error: #error unknown ACPI_MACHINE_WIDTH
   #error unknown ACPI_MACHINE_WIDTH
    ^~~~~
  actypes.h:207:9: error: unknown type name ‘acpi_native_uint’
   typedef acpi_native_uint acpi_size;
           ^~~~~~~~~~~~~~~~
  actypes.h:617:3: error: unknown type name ‘acpi_io_address’
     acpi_io_address pblk_address;
     ^~~~~~~~~~~~~~~

This likely went undetected with glibc builds since glibc
indirectly pulls __BITS_PER_LONG from the linux headers
through a standard header. For musl, this is not the case.

Instead, use BITS_PER_LONG to fix the build.

Signed-off-by: Daniel Sabogal <dsabogalcc@gmail.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>