In binutils 2.29, the behavior of the ADR instruction changed so that 1 is
added to the address of a Thumb function (previously nothing was added). This
allows the loaded address to be passed to a BLX instruction and the correct
mode change will occur.

See: https://sourceware.org/bugzilla/show_bug.cgi?id=21458

By using adr with a label that isn't annotated as a thumb function,
we avoid the new behaviour in binutils 2.29 and get the same behaviour
as in prior releases, and as in other assemblers (ms armasm.exe,
clang's built in assembler) - an idea that Janne Grunau came up with.
Signed-off-by: Martin Storsjö <martin@martin.st>