gmp: Don't parse PKCS1 v1.5 RSA signatures to verify them

gmp: Don't parse PKCS1 v1.5 RSA signatures to verify them

Instead we generate the expected signature encoding and compare it to the
decrypted value.

Due to the lenient nature of the previous parsing code (minimum padding
length was not enforced, the algorithmIdentifier/OID parser accepts arbitrary
data after OIDs and in the parameters field etc.) it was susceptible to
Daniel Bleichenbacher's low-exponent attack (from 2006!), which allowed
forging signatures for keys that use low public exponents (i.e. e=3).

Since the public exponent is usually set to 0x10001 (65537) since quite a
while, the flaws in the previous code should not have had that much of a
practical impact in recent years.

Fixes: CVE-2018-16151, CVE-2018-16152