Copy over the ASM_DECLARE_FUNCTION_SIZE macro from linux64.h. This macro
declares the proper size of a function. Without this macro recent GNU as will
complain about with:
'Error: .size expression for main does not evaluate to a constant.'

Up to now we produce this:

.L.main:

	....

.size main, .-main

With the macro defined the output is this:

.L.main:

	....

.size main,.-.L.main

This affects only the 64-bit compiler.
Tested with world and kernel on both, 32 and 64-bit powerpc.