exec: move memory access declarations to a common header, inline *_phys functions

For now, this reduces the text size very slightly due to the newly-added
inlining:

   text size before: 9301965
   text size after: 9300645

Later, however, the declarations in include/exec/memory_ldst.inc.h will be
reused for the MemoryRegionCache slow path functions.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>