sh: Simplify mksyntax and make it fit for cross-compiling.

sh: Simplify mksyntax and make it fit for cross-compiling.

Now it outputs fixed files, which use constants provided by the C standard
library to determine appropriate values for the target machine.

Before, mksyntax inspected the host machine which resulted in subtle
breakage if e.g. char is signed on the host and unsigned on the target such
as when cross-compiling on x86 for ARM.

Tested using -funsigned-char on amd64. Compiling build-tools without it and
sh itself with it causes various tests to fail without this change but not
with this change. With consistent -funsigned-char, tests pass with or
without this change.

The mksyntax program could be removed and syntax.c and syntax.h committed to
the repository.

Submitted by:	Christoph Mallon
MFC after:	2 weeks