Ptrace syscall testing fully support on Arm64

[lubinszARM]

I plan to enable all syscall tests on Arm64.
Currently, there are several random problemes left:

• udp_socket_test_runsc_ptrace_hostnet
• random memory barrier issue in mmap testing
• runtime fails with missing stack maps in race mode
  Description: In race mode, when calling the go function in asm code, there will be an missing stack maps issue.
• Since golang 1.15.2 has fixed the arm64 overflow issue, we need to update go_version into 1.15.2 in gVisor.
• A random seccomp bpf fail issue in syscall testing

Currently known golang issues:

• overflow issue - need golang 1.15.2+
  https://go-review.googlesource.com/c/go/+/233097

/assign @lubinszARM

[lubinszARM]

Test info (2020-05-27):
1,
uname -a
Linux entos-thunderx2-04 5.5.0-rc7+ #8 SMP Tue May 26 00:00:18 EDT 2020 aarch64 aarch64 aarch64 GNU/Linux

2, test command:
bblu@entos-thunderx2-04:~/gvisor/gvisor$ bazel test --test_tag_filters=runsc_ptrace //test/syscalls/...

[lubinszARM]

cc @avagin @amscanne