[musl] PCC unable to build musl 1.2.0 (and likely earlier)

[John Arnold <iohannes.eduardus.arnold@gmail.com>]

With an i386 PCC 1.2.0.DEVEL built from source from
http://pcc.ludd.ltu.se/ftp/pub/pcc/pcc-20200510.tgz, I was unable to
build an i386 musl 1.2.0. The compiler first hits this error:

./include/limits.h:10: error: bad charcon

This line was the only change made in commit cdbbcfb8f5d, but it has a
lengthy commit message about the proper way of determining CHAR_MIN
and CHAR_MAX.

Reverting that change fixes the issue with limits.h, but PCC then runs
into another problem:

src/complex/catan.c, line 105: operands of = have incompatible types
src/complex/catan.c, line 105: cannot recover from earlier errors: goodbye!

catan.c only has one change that could possibly be relevant:
10e4bd3780050e75b72, which fixed a hack labeled FIXME.

Undoing this change results in hitting yet more errors:
/tmp/ctm.AkDmnc: Assembler messages:
/tmp/ctm.AkDmnc:50: Error: bad register name `%%ax'

Which are beyond my ability to debug on my own. Happy to help with the
hunt though. I'm not on the musl mailing list, but please CC me on the
replies to this.
-- John