apacheGH-41020: [C++] Introduce portable compiler assumptions (apache…

…#41021)

### Rationale for this change

Allow portable use of the "assumption" feature of modern GCC, Clang, and MSVC compilers.

### What changes are included in this PR?

 - Documenting of macros in `macros.h`
 - Addition of `ARROW_COMPILER_ASSUME`
 - One simple change where this macro simplifies the existing code while keeping the common case of a loop branch dependent only on local variables
 
### Are these changes tested?

By existing tests.
* GitHub Issue: apache#41020

Authored-by: Felipe Oliveira Carvalho <felipekde@gmail.com>
Signed-off-by: Felipe Oliveira Carvalho <felipekde@gmail.com>

cpp/src/arrow/util/macros.h

@@ -71,7 +71,6 @@
 #define ARROW_ARG_UNUSED(x) x
 #else
 #define ARROW_ARG_UNUSED(x)
-#endif
 #if defined(__GNUC__) // GCC and compatible compilers (clang, Intel ICC)
 #define ARROW_NORETURN __attribute__((noreturn))
 #define ARROW_NOINLINE __attribute__((noinline))