MFC: r272256

  Switch from lib/libstand to sys/boot/libstand32.
MFC: r272258
  - Cosmetic changes.
  - Reduce diffs against i386.

MFC: 272259
  MFi386: Enable QUOTA, PRINTF_BUFR_SIZE and puc.
MFC: r268475 (by imp)

  Make SERIAL support optional again. Enable it for i386 because a huge
  percentage of machines has a 16550. Disable it for pc98 since only a
  tiny fraction of them have one.
MFC: r268474 and r268476 (by imp)
  - Merge the clang support from i386.
  - Compile boot2 with clang on pc98.

MFC: r272250 (by nyan)
  - Reduce diffs against i386.
MFC r271946 and r272595:
Improve transmit sending offload, TSO, algorithm in general. This
change allows all HCAs from Mellanox Technologies to function properly
when TSO is enabled. See r271946 and r272595 for more details about
this commit.

Sponsored by:	Mellanox Technologies