tools: Rename __fallthrough to fallthrough

tools: Rename __fallthrough to fallthrough

Rename the fallthrough attribute to better align with the kernel
version.  Copy the definition from include/linux/compiler_attributes.h
including the #else clause.  Adding the #else clause allows the tools
compiler.h header to drop the check for a definition entirely and keeps
both definitions together.

Change any __fallthrough statements to fallthrough anywhere it was used
within perf.

This allows other tools to use the same key word as the kernel.

Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Miguel Ojeda <ojeda@kernel.org>
Cc: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Tom Rix <trix@redhat.com>
Signed-off-by: Liam Howlett <Liam.Howlett@oracle.com>