kselftest/arm64: Use syscall() macro over nolibc my_syscall()

kselftest/arm64: Use syscall() macro over nolibc my_syscall()

The my_syscall*() macros are internal implementation details of nolibc.
Nolibc also provides the regular syscall(2), which is also a macro
and directly expands to the correct my_syscall().

Use syscall() instead.

As a side-effect this fixes some return value checks, as my_syscall()
returns the raw value as set by the kernel and does not set errno.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Acked-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Will Deacon <will@kernel.org>