[compiler-rt] Fix ptrace interceptor for aarch64

This patch fixes the ptrace interceptor for aarch64. The PTRACE_GETREGSET
ptrace syscall with with invalid memory might zero the iovec::iov_base
field and then masking the subsequent check after the syscall (since it
will be 0 and it will not trigger an invalid access). The fix is to copy
the value on a local variable and use its value on the checks.

The patch also adds more coverage on the Linux/ptrace.cc testcase by addding
check for PTRACE_GETREGSET for both general and floating registers (aarch64
definitions added only).

llvm-svn: 251331