Clarify that effective-targets 'exceptions' and 'exceptions_enabled' are orthogonal

In Subversion r268025 (Git commit 3f21b8e3)
"Add dg-require-effective-target exceptions", effective-target 'exceptions'
was added, which "says that AMD GCN does not support [exception handling]".

In Subversion r279246 (Git commit a9046e98)
"MSP430: Add -fno-exceptions multilib", effective-target 'exceptions_enabled'
was added "to check if the testing configuration supports exceptions".  Testing
"if exceptions are unsupported or disabled (e.g. by passing -fno-exceptions)"
works as expected if exception handling is disabled at the front-end level
('-fno-exceptions'; the "exceptions are [...] disabled" case):

    exceptions_enabled2066068.cc: In function ‘void foo()’:
    exceptions_enabled2066068.cc:3:27: error: exception handling disabled, use ‘-fexceptions’ to enable

However, effective-target 'exceptions_enabled' additionally assumes that
"If exceptions aren't supported [by the target], then they're not enabled".
This is not correct: it's not unlikely that, in presence of explicit/implicit
'-fexceptions', exception handling code gets fully optimized away by the
compiler, and therefore effective-target 'exceptions_enabled' test cases may
PASS even for targets that don't support effective-target 'exceptions'; these
two effective-targets are orthogonal concepts.

(For completeness: code with trivial instances of C++ exception handling may
translate into simple '__cxa_allocate_exception', '__cxa_throw' function calls
without requiring any back end-level "exceptions magic", and then trigger
unresolved symbols at link time, if these functions are not available.)

This change only affects GCN, as that one currently is the only target declared
as not supporting effective-target 'exceptions'.

	gcc/
	* doc/sourcebuild.texi (Effective-Target Keywords): Clarify that
	effective-target 'exceptions' and 'exceptions_enabled' are
	orthogonal.
	gcc/testsuite/
	* lib/gcc-dg.exp (gcc-dg-prune): Clarify effective-target
	'exceptions_enabled'.
	* lib/target-supports.exp
	(check_effective_target_exceptions_enabled): Don't consider
	effective-target 'exceptions'.
	libstdc++-v3/
	* testsuite/lib/prune.exp (libstdc++-dg-prune): Clarify
	effective-target 'exceptions_enabled'.