Delete malloc.h and tgmath.h from libc.

Delete malloc.h and tgmath.h from libc.

This malloc.h might get confused with the private malloc.h in our
libc.  C programs should #include <stdlib.h> for malloc().

This tgmath.h has no useful content, and never worked because
complex.h is missing.

Touch build.lua (by deleting some whitespace) so the *.h globs see
the deletions.