loader: add support for hybrid PMBR for GPT partition table

Note hybrid table is nor really UEFI specification compliant.

Sample hybrid partition table:

::mbr

Format: unknown
Signature: 0xaa55 (valid)
UniqueMBRDiskSignature: 0

PART TYPE ACTIVE STARTCHS ENDCHS SECTOR NUMSECT
0 EFI_PMBR:0xee 0 1023/254/63 1023/254/63 1 409639
1 0xff 0 1023/254/63 1023/254/63 409640 978508408
2 FDISK_EXT_WIN:0xc 0 1023/254/63 1023/254/63 978918048 31250000
3 0xff 0 1023/254/63 1023/254/63 1010168048 32