Fix conditional compilation of sys/mman.h

See merge request redox-os/relibc!201

Aarch64 prep redux

See merge request !202

Without this the relibc tests would break. With this the relibc tests
pass and formerly breaking builds (such as uutils -> onig_sys) pass.

Typically with libc implementations, wchar_t and co are either defined
entirely by the libc or, under libc's arrangement, by headers supplied
by the compiler.

Things like dlmalloc in relibc need these definitions from relibc itself
and that's already already furnished by relibc's stddef.h.

These additional definitions here are redundant and collide with
compiler headers - for example: onig_sys (something that uutils depends
on) breaks. Instead, this patch makes the compiler headers define
things appropriately.

For AArch64, the ring crate depends on the presence of this header and a definition
of getauxval.

The pre-index operator ('!') was missing at the end of the stp
instruction.

As a result, the stack pointer wasn't updated after the
store of the 64-bit pair and the stored values were basically lost when
follow on code used the stack for later store ops.

tests: Macro based error handling

See merge request redox-os/relibc!195

Implement swab

See merge request !187

implements wcsrchr from wchar.h

See merge request !197

add calloc integer overflow check

See merge request !188

Change LONG_BIT definition, fixes #148

Closes #148

See merge request !198

Turns the results of these macros from compound to regular statements using the old `do { ... } while(0)` trick. Must have for function-like macros.

realpath: Fixing undefined behaviour in the second test. If the call fails the resolved_name argument cannot be used for error checking because its state is undefined by SUSv2.
pipe: Changing the order of close and write error handling code. Errors in close could overwrite errno after write errors, returning incorrect error messages.
gmtime: Removed duplicate checks
Other fixes for fseek, rename, mktime, putwchar

Implements wcscspn function from wchar.h

See merge request !196

This will allow us to redefine the exit function.

For example:
```
#define exit(code) { \
    fprintf(stderr, "%s:%d: exit(%s) in function ‘%s’\n",
        __FILE__, __LINE__, #code, __func__); \
    _exit(code); \
}
```

General test cleanups

See merge request !193

unistd: Generate correct C defines for PATH_SEPARATOR

See merge request redox-os/relibc!194

When the execution reaches the end of the main functions, they implicitly return a successful status.

tests: set C11, enable pedantic warnings, fix GCC and Clang warnings

See merge request !192

ctype: Implement _tolower, _toupper

See merge request redox-os/relibc!191