arm64: use "b" to branch to start_xen

arm64: use "b" to branch to start_xen

The cbz instruction has range limitation. When compiled with gcov
support the object is larger so cbz can't handle that anymore. The error
message is like:

aarch64-linux-gnu-ld    -EL  -T xen.lds -N prelink.o \
    /local/work/xen.git/xen/common/symbols-dummy.o -o /local/work/xen.git/xen/.xen-syms.0
prelink.o: In function `launch':
/local/work/xen.git/xen/arch/arm/arm64/head.S:602:(.text+0x408): relocation truncated to fit: R_AARCH64_CONDBR19 against symbol `start_xen' defined in .init.text section in prelink.o

Use "b" instead.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reviewed-by: Julien Grall <julien.grall@arm.com>