Clang has its own limits.h which is ahead of ours on the inclusion
path. This header uses include_next to include our header, but only
in hosted mode. This means that in freestanding mode we don't get our
limits.h macro definitions, including LONG_BIT. This ends up causing
our signal.h to produce errors when included in freestanding mode on
32-bit platforms.
Fix the errors by replacing usage of LONG_BIT with (8 * sizeof(long))
in the signal headers.
Change-Id: I18ec7b6876d5f862beae09f0c011128eef97c869
08b968b282