RFD: inline hooks

[Joern Rennecke]

For the rtl passes, architecture target macros are not much of an issue
with regards to executable code modularity: since the rtl passes are
deeply interwoven with the insn-*.c files, we might as well compile one
specialized copy of the rtl passes for each target architecture.

Another argument against leaving the macros are their often ill-defined
interface types and the call-by-name semantics that make all the identifiers
in scope at the call site a potential interface.

We could avoid the latter problems without sacrificing the speed that we
get from target-specific code by replacing the target macro with an
inline hook.  E.g. consider HARD_REGNO_MODE_OK.  We could have $tm_file
define TARGET_HARD_REGNO_MODE_OK as a static inline function, or #define it
as the name of a static inline function somewhere else in $tm_file.
The function's address will be in TARGET_INITIALIZER, and thus type
checking on the function definition will be done.

But a file that includes tm.h will be able to use the function
TARGET_HARD_REGNO_MODE_OK directly, which can then be inlined, thus
giving type safety without performance penalty.