Support compiling libjemalloc5 against musl

Musl in the Android platform is configured to use libjemalloc5 by
default as the built-in malloc causes a segfault when running aprotoc.

Fix a build error that assumes pthread_t is an unsigned long when
initializing it to zero.

Adjust the host configuration to work with musl.

Bug: 190084016
Test: m out/soong/host/linux-x86/bin/aprotoc && out/soong/host/linux-x86/bin/aprotoc
Change-Id: I5b5d5226744817b8705d58f6b71cf6c62b128eee
3 files changed