at91sam7x: force inlining of all the UART ISR functions.

NOTE: this is not just an improvement (that is worth to be applied
anyway), it's also a workaround for a GCC bug
(http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41999): the compiler
corrupts the stack pointer and the frame pointer when calling functions
with at least one parameter inside ISR functions, declared with
__attribute__((interrupt)).

git-svn-id: https://src.develer.com/svnoss/bertos/trunk@3465 38d2e660-2303-0410-9eaa-f027e97ec537