translate-all: Mark map_exec() with the 'unused' attribute

translate-all: Mark map_exec() with the 'unused' attribute

Mark map_exec() with the 'unused' attribute to avoid '-Wunused-function'
warnings on clang 3.4 or later. This means we don't need to mark it
'inline', which is what we were previously using to suppress the warning
(a trick which only works with gcc, not clang).

Signed-off-by: SeokYeon Hwang <syeon.hwang@samsung.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
[PMM: tweaked comment message a little]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>