Not all toolchains use pc relative addresses for global variables.
Apparently the gcc 8.3.0 YOCTO toolchain uses absolute addresses.
This means can't simply return 0 for global_variable_offset() but
instead have to calculate the offset between the compile addresses
for global variables and their runtime address.
We do this by getting the address of a global variable pc relative
explicitely in assembly and substracting that address from the
location the C compiler thinks they are.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>