[musl][bootloader] Use compiler-provided standard headers

<stdalign.h>, <stdarg.h>, <stdbool.h>, <stddef.h>, <stdint.h>
are all provided by the compiler and should not be replaced here.

The bootloader code's local versions were sloppy in what names
got defined where, so the code needed some #include additions
and type adjustments.

TC-32 #done

Change-Id: Ib24ba7b5072e99ec9302a6994d63ad5e16233151

[bootloader] Initial import of gigaboot

Original source:
https://fuchsia.googlesource.com/gigaboot20x6/
Upstream commit:
80eef665f1e248eafe48f9b430fdae702ece20c1

Change-Id: I1be9ad20831e0d790937446febc28755943884a5