-O2 gcc flag breaks armhf build

[Raimo33]

When compiling for an arm 32bit (armhf) target, ./configure yields this error:

checking for unaligned access to int... ccan/tools/configurator/configurator: Test for HAVE_UNALIGNED_ACCESS did not compile:
configuratortest.c: In function 'main':
configuratortest.c:8:22: error: array subscript 'int[0]' is partly outside array bounds of 'char[4]' [-Werror=array-bounds]
     8 |         return *x == *y;
        |                      ^~
configuratortest.c:5:11: note: at offset 1 into object 'pad' of size 4
     5 |      char pad[sizeof(int *) * 1];
        |           ^~~
cc1: all warnings being treated as errors 

This only happens when using the -O2 flag and above. -O1 compiles fine.

to reproduce, you can use a cross compiler (GCC) and these flags: "-O2 -march=armv7-a -mfpu=vfpv3-d16 -mfloat-abi=hard"

[jgriffiths]

FYI I worked around this issue in wally with: ElementsProject/libwally-core@745bc55

See the commit for a description of why the workaround is required.