build: fix "transform-o-to-shared-lib-inner" for armv6

GCC built-in functions provide advanced things like prefetch,
expected, synchronization...

GCC implements these functions in 2 ways:
A If the function implementation is simple, some assembly instructions
  are used
B If the function implementation is complicated, GCC calls a function
  in the GCC runtime library (-lgcc)

Some functions can be implemented in a simple way on armv7 targets,
but need a complicated implementation on armv6 targets. So for armv6
targets, a call to the GCC library is used.

For example: "__sync_synchronize":

echo "void test() { __sync_synchronize(); }" > test.c
./arm-linux-androideabi-gcc -march=armv6k test.c -c -mthumb -o armv6k.o
./arm-linux-androideabi-gcc -march=armv7-a test.c -c -mthumb -o armv7-a.o

armv6k.o:
b580        push  {r7, lr}
af00        add r7, sp, #0
f7ff fffe   bl  0 <__sync_synchronize>
46bd        mov sp, r7
bd80        pop {r7, pc}

armv7-a:
b480        push  {r7}
af00        add r7, sp, #0
f3bf 8f5f   dmb sy
46bd        mov sp, r7
f85d 7b04   ldr.w r7, [sp], #4
4770        bx  lr

Normally the "-lgcc" is enabled by default, but some libs, in Lollipop,
disable it (-nodefaultlibs or -nostdlib). For this reason GCC can't
resolve the call to the built-in functions.

"-lgcc" must be added explicitly.

Change-Id: I0014052dd97229c5feacac12e9b9af2c344f4c0e
Signed-off-by: Marco Milanese <marcomilanese7@gmail.com>