labels, outelf: remove casts for allocations

labels, outelf: remove casts for allocations

Remove casts from allocations.  This is simply Not How To Do Things:
every cast carries a potential risk of being a toxic type misuse
(e.g. pointer as integer) and so any unnecessary cast is actively
harmful.

Note that a lot of allocations here are completely unnecessary: the
core code now guarantees that all filenames are permanently allocated
for the duration of the assembly, and so should be turned into const
char * without any further allocation.  Any remaining malloc+strcpy
should be turned into nasm_strdup(), and nasm_new[n]() used whereever
possible.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>