[pcbios] Allow for larger-than-20-byte buffers in e820mangler.S

[pcbios] Allow for larger-than-20-byte buffers in e820mangler.S

Although the E820 API allows for a caller to provide only a 20-byte
buffer, there exists at least one combination (HP BIOS, 32-bit WinPE)
that relies on information found only in the "extended attributes"
field, which requires a 24-byte buffer.

Allow for up to a 64-byte E820 buffer, in the hope of coping with
future idiocies like this one.