By default, C function prototypes declared in headers are visible,
so there is no need to declare them as 'extern' functions.
Remove this redundancy in a single bulk commit; do not modify:

  - meson.build (used to check function availability at runtime)
  - pc-bios/
  - libdecnumber/
  - tests/
  - *.c

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20230605175647.88395-5-philmd@linaro.org>