eal/unix: make stack dump signal safe

eal/unix: make stack dump signal safe

rte_dump_stack() needs to be usable in situations when a bug is
encountered and from signal handlers (such as SEGV).

Glibc backtrace_symbols() calls malloc which makes it
dangerous in a signal handler that is handling errors that maybe
due to memory corruption. Additionally, rte_log() is unsafe because
syslog() is not signal safe; printf() is also documented as
not being safe.

This version formats message and uses writev for each line in a manner
similar to what glibc version of backtrace_symbols_fd() does. The
FreeBSD version of backtrace_symbols_fd() is not signal safe.

Sample output:

0: ./build/app/dpdk-testpmd (rte_dump_stack+0x2b) [560a6e9c002b]
1: ./build/app/dpdk-testpmd (main+0xad) [560a6decd5ad]
2: /lib/x86_64-linux-gnu/libc.so.6 (__libc_start_main+0xcd) [7fd43d3e27fd]
3: ./build/app/dpdk-testpmd (_start+0x2a) [560a6e83628a]

Bugzilla ID: 929

Acked-by: Morten Brørup <mb@smartsharesystems.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Reviewed-by: David Marchand <david.marchand@redhat.com>