scudo/standalone: prepare for enabling format string checking

Move __attribute__((format)) to the function declarations in the header file.
It's almost pointless in the source file.
But disable the warning  with -Wno-format for now
since there is a number of existing warnings.

Depends on D107984.

Reviewed By: vitalybuka

Differential Revision: https://reviews.llvm.org/D108014