Fix compilation of crtbegin.c and pthread_debug.cpp.

Because there was no default := for the aarch64 libc_crt_target_cflags,
the += was causing libc_crt_target_cflags to be recursively-defined
variable, which meant that when we were compiling crtbegin.c LOCAL_PATH
would be bionic/tests/ and we'd have -Ibionic/tests/include/ and find
none of our include files.

Also fix linking of pthread_debug.cpp, at least in the disabled mode.
The enabled mode was already broken for all architectures, and continues
to be broken after this change. It's been broken for long enough that
we might want to just remove it...

(aarch64 is using the FSF linker where arm uses the gold linker.)

Change-Id: I7db2e386694f6933db043138e6e97e5ae54d4174