perf doc: Add support for KBUILD_BUILD_TIMESTAMP

perf doc: Add support for KBUILD_BUILD_TIMESTAMP

When building man pages from a Git checkout, we consistently set the
man page date based on when the input was last changed.  Otherwise, it
defaults to the build time, which is not reproducible.

Allow the date to be set through the KBUILD_BUILD_TIMESTAMP variable,
as for timestamps in the kernel itself.

Signed-off-by: Ben Hutchings <benh@debian.org>
Acked-by: Ian Rogers<irogers@google.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Salvatore Bonaccorso <carnil@debian.org>
Link: https://lore.kernel.org/r/ZF/1F1P+b9qZ/vVH@decadent.org.uk
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>