ftrace: Have recordmcount use w8 to read relp->r_info in arm64_is_fake_mcount

On little endian system, Use aarch64_be(gcc v7.3) downloaded from
linaro.org to build image with CONFIG_CPU_BIG_ENDIAN = y,
CONFIG_FTRACE = y, CONFIG_DYNAMIC_FTRACE = y.

gcc will create symbols of _mcount but recordmcount can not create
mcount_loc for *.o.
aarch64_be-linux-gnu-objdump -r fs/namei.o | grep mcount
00000000000000d0 R_AARCH64_CALL26  _mcount
...
0000000000007190 R_AARCH64_CALL26  _mcount

The reason is than funciton arm64_is_fake_mcount can not work correctly.
A symbol of _mcount in *.o compiled with big endian compiler likes:
00 00 00 2d 00 00 01 1b
w(rp->r_info) will return 0x2d instead of 0x011b. Because w() takes
uint32_t as parameter, which truncates rp->r_info.

Use w8() instead w() to read relp->r_info

Link: https://lkml.kernel.org/r/20210222135840.56250-1-chenjun102@huawei.com

Fixes: ea0eada4

 ("recordmcount: only record relocation of type R_AARCH64_CALL26 on arm64.")
Acked-by: Will Deacon <will@kernel...>