x29 (FP) is a callee saved register and should be restored on
return. Instead of backing up x29 and restoring it here, back up
sp in a register that we are allowed to overwrite.

This fixes crashes in checkasm on aarch64 since f1b3e131.
For some reason, gcc builds didn't crash, but clang builds do.
Signed-off-by: Martin Storsjö <martin@martin.st>