ANDROID: mm: add generic __va_function and __pa_function

We use non-canonical CFI jump tables with CONFIG_CFI_CLANG, which
means the compiler replaces function address references with the
address of the function's CFI jump table entry. This results in
__pa_symbol(function), for example, returning the physical address
of the jump table entry, which can lead to address space confusion
since the jump table itself points to a virtual address.

This change adds generic definitions for __pa/va_function, which
architectures that support CFI can override.

Bug: 166220312
Change-Id: If4a162de7854547d8437bf59be03eb6d16220832
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>