Fix libcore compiling against musl

Fix various issues when compiling host modules against musl:

Musl only provides the posix version of strerror_r that returns int.

Update sys/poll.h and sys/signal.h includes to point to the correct
locations.

Bug: 190084016
Test: m USE_HOST_MUSL=true
Change-Id: If6c9c31c935767e82508f8a469bdb499c7a96115
8 files changed