debuggerd_handler: don't use snprintf in handler.

snprintf isn't safe to call in the linker after initialization, because
it uses MB_CUR_MAX which is implemented via pthread_getspecific, which
uses TLS slots shared with libc. If the TLS slots are assigned in a
different order between libc.so and the linker, MB_CUR_MAX will
evaluate to an incorrect value, and lead to snprintf doing bad things.

Switch to __libc_format_buffer.

Bug: http://b/35367169
Test: debuggerd -b `pidof zygote`
Change-Id: I9d315cf63e5f3fd2f4545d6e3f707cdbe94ec606