Add support for building m4 against musl

Generate linux_musl headers with
soong/regen.sh --musl_sysroot /tmp/musl-sysroot

Bug: 190084016
Test: m out/soong/host/linux-x86/bin/m4 with musl
Change-Id: I6f44a99c2fb547d426be829ccd226967e1551a91
28 files changed