exec: Rename NEED_CPU_H -> COMPILING_PER_TARGET

exec: Rename NEED_CPU_H -> COMPILING_PER_TARGET

'NEED_CPU_H' guard target-specific code; it is defined by meson
altogether with the 'CONFIG_TARGET' definition. Rename NEED_CPU_H
as COMPILING_PER_TARGET to clarify its meaning.

Mechanical change running:

 $ sed -i s/NEED_CPU_H/COMPILING_PER_TARGET/g $(git grep -l NEED_CPU_H)

then manually add a /* COMPILING_PER_TARGET */ comment
after the '#endif' when the block is large.

Inspired-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20240322161439.6448-4-philmd@linaro.org>