perf tools: Remove subcmd dependencies on strbuf

perf tools: Remove subcmd dependencies on strbuf

Introduce and use new astrcat() and astrcatf() functions which replace
the strbuf functionality for subcmd.

For now they duplicate strbuf's die-on-allocation-error policy.

Signed-off-by: Josh Poimboeuf <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Link: http://lkml.kernel.org/r/957d207e1254406fa11fc2e405e75a7e405aad8f.1450193761.git.jpoimboe@redhat.com
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>