Makefile - use -gddb2, not -gddb3 due to issues with GDB crashing. (b…

…etaflight#12971)

Makefile - use gddb2, not gddb3 due to issues with GDB crashing.

The cause seems to be the macro debugging information which when generated by the compiler, apparently incorrectly, causes GDB to use excessive amounts of CPU and crash.

Error:
"gdb/utils.c:717\
: internal-error: virtual memory exhausted: can't allocate 4064
bytes.\nA problem internal to GDB ha\
s been detected,\nfurther debugging may prove unreliable."

Reference:

* https://sourceware.org/bugzilla/show_bug.cgi?id=28219#c15
* microsoft/vscode-cpptools#9219 (comment)

Note this crashing behavior is observed with `GCC > 9.3.1`, `9.3.1` itself is OK.

Makefile

@@ -145,10 +145,10 @@ ifeq ($(DEBUG),GDB)
 OPTIMISE_DEFAULT      := -Og
 
 LTO_FLAGS             := $(OPTIMISE_DEFAULT)
-DEBUG_FLAGS            = -ggdb3 -gdwarf-5 -DDEBUG
+DEBUG_FLAGS            = -ggdb2 -gdwarf-5 -DDEBUG
 else
 ifeq ($(DEBUG),INFO)
-DEBUG_FLAGS            = -ggdb3
+DEBUG_FLAGS            = -ggdb2
 endif
 OPTIMISATION_BASE     := -flto -fuse-linker-plugin -ffast-math -fmerge-all-constants
 OPTIMISE_DEFAULT      := -O2