ArmPkg/CompilerIntrinsicsLib: fix GCC8 warning for __aeabi_memcpy aliases

ArmPkg/CompilerIntrinsicsLib: fix GCC8 warning for __aeabi_memcpy aliases

This was the warning (shown for __aeabi_memcpy, __aeabi_memcpy4 and
__aeabi_memcpy8):

  ArmPkg/Library/CompilerIntrinsicsLib/memcpy.c:42:6:
  error: '__aeabi_memcpy8' alias between functions of incompatible types
    'void(void*, const void *, size_t)'
      {aka 'void(void *, const void *, unsigned int)'}
    and 'void *(void *, const void *, size_t)'
      {aka 'void *(void *, const void *, unsigned int)'} [-Werror=attribute-alias]
  void __aeabi_memcpy8(void *dest, const void *src, size_t n);
  ArmPkg/Library/CompilerIntrinsicsLib/memcpy.c:19:7: note: aliased declaration here
    void *__memcpy(void *dest, const void *src, size_t n)

The problem is the different return type (void vs void*). So reshuffle
the code so the prototypes match between the aliases.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Michael Zimmermann <sigmaepsilon92@gmail.com>
[ardb: change prototype of internal __memcpy() and drop extra wrapper]
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>