Remove redundant casts in GC_generic_or_special_malloc and similar

Remove redundant casts in GC_generic_or_special_malloc and similar
(refactoring)

* mallocx.c (GC_generic_or_special_malloc,
GC_generic_malloc_ignore_off_page): Remove redundant casts for
"lb" argument; remove redundant parentheses.
* fnlz_mlc.c (GC_finalized_malloc): Likewise.
* malloc.c (GC_generic_malloc, GC_malloc_uncollectable): Likewise.
* malloc.c (GC_malloc_uncollectable): Remove redundant cast of
returned value.
* mallocx.c (GC_malloc_ignore_off_page,
GC_malloc_atomic_ignore_off_page): Likewise.