package/go: security bump to 1.15.5

package/go: security bump to 1.15.5

Fixes the following security issues:

- math/big: panic during recursive division of very large numbers

  A number of math/big.Int methods (Div, Exp, DivMod, Quo, Rem, QuoRem, Mod,
  ModInverse, ModSqrt, Jacobi, and GCD) can panic when provided crafted
  large inputs.  For the panic to happen, the divisor or modulo argument
  must be larger than 3168 bits (on 32-bit architectures) or 6336 bits (on
  64-bit architectures).  Multiple math/big.Rat methods are similarly affected.

  crypto/rsa.VerifyPSS, crypto/rsa.VerifyPKCS1v15, and crypto/dsa.Verify may
  panic when provided crafted public keys and signatures.  crypto/ecdsa and
  crypto/elliptic operations may only be affected if custom CurveParams with
  unusually large field sizes (several times larger than the largest
  supported curve, P-521) are in use.  Using crypto/x509.Verify on a crafted
  X.509 certificate chain can lead to a panic, even if the certificates
  don’t chain to a trusted root.  The chain can be delivered via a
  crypto/tls connection to a client, or to a server that accepts and
  verifies client certificates.  net/http clients can be made to crash by an
  HTTPS server, while net/http servers that accept client certificates will
  recover the panic and are unaffected.

  Moreover, an application might crash invoking
  crypto/x509.(*CertificateRequest).CheckSignature on an X.509 certificate
  request or during a golang.org/x/crypto/otr conversation.  Parsing a
  golang.org/x/crypto/openpgp Entity or verifying a signature may crash.
  Finally, a golang.org/x/crypto/ssh client can panic due to a malformed
  host key, while a server could panic if either PublicKeyCallback accepts a
  malformed public key, or if IsUserAuthority accepts a certificate with a
  malformed public key.

  Thanks to the Go Ethereum team and the OSS-Fuzz project for reporting
  this.  Thanks to Rémy Oudompheng and Robert Griesemer for their help
  developing and validating the fix.

  This issue is CVE-2020-28362 and Go issue golang.org/issue/42552.

- cmd/go: arbitrary code execution at build time through cgo

  The go command may execute arbitrary code at build time when cgo is in
  use.  This may occur when running go get on a malicious package, or any
  other command that builds untrusted code.

  This can be caused by malicious gcc flags specified via a #cgo directive,
  or by a malicious symbol name in a linked object file.

  Thanks to Imre Rad and to Chris Brown and Tempus Ex respectively for
  reporting these issues.

  These issues are CVE-2020-28367 and CVE-2020-28366, and Go issues
  golang.org/issue/42556 and golang.org/issue/42559 respectively.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>