Hwasan InitPrctl check for error using internal_iserror

When adding this function in https://reviews.llvm.org/D68794 I did not
notice that internal_prctl has the API of the syscall to prctl rather
than the API of the glibc (posix) wrapper.

This means that the error return value is not necessarily -1 and that
errno is not set by the call.

For InitPrctl this means that the checks do not catch running on a
kernel *without* the required ABI (not caught since I only tested this
function correctly enables the ABI when it exists).
This commit updates the two calls which check for an error condition to
use `internal_iserror`.  That function sets a provided integer to an
equivalent errno value and returns a boolean to indicate success or not.

Tested by running on a kernel that has this ABI and on one that does
not.  Verified that running on the kernel without this ABI the current
code prints the provided error message and does not attempt to run the
program.  Verified that running on the kernel with this ABI the current
code does not print an error message and turns on the ABI.
All tests done on an AArch64 Linux machine.

Reviewed By: eugenis

Differential Revision: https://reviews.llvm.org/D94425