arm64: Fix COMPAT_FREEBSD32.

The ENTRY() macro was modified by commit
28d945204ea1014d7de6906af8470ed8b3311335 to add an optional NOP instruction
at the beginning of the function. It is of course an arm64 instruction, so
unsuitable for the 32bits sigcode. So just use EENTRY() instead for
aarch32_sigcode. This should fix receiving signals when running 32bits
binaries on FreeBSD/arm64.

MFC After: 1 week