BUILD: makefile: enable backtrace by default on musl

BUILD: makefile: enable backtrace by default on musl

The reason musl builds was not producing exploitable backtraces was
that the toolchain used appears to automatically omit the frame pointer
at -O2 but leaves it at -O0. This patch just makes sure to always append
-fno-omit-frame-pointer to the BACKTRACE cflags and enables the option
with musl where it now works. This will allow us to finally get
exploitable traces from docker images where core dumps are not always
available.

(cherry picked from commit f499fa3dcd24b5a17ed97842f5e867bd37739754)
[wt: this should be progressively backported to 3.0 or maybe even 2.8
 since a few users have already reported hard-to-debug issues in Docker]
Signed-off-by: Willy Tarreau <w@1wt.eu>