Matching contrib/gcc/gcc.c commit of rev 1.23, remove misleading comment
about 'STANDARD_EXEC_PREFIX'. It is used now and is needed if one does not
set 'MD_EXEC_PREFIX'.

Do not define a 'MD_EXEC_PREFIX'. It is not needed, not used in the
cross case, and just ends up causing "/usr/libexec" being added to the
library search path.