Porting/Bitcoin Core

Finished

• Big endian support
• build: disable optional use of gmp in internal secp256k1 build
• add powerpc build support for openssl lib
• tests: Switch one of the Travis jobs to an unsigned char environment (-funsigned-char)
• contrib: use newer config.guess & config.sub in install_db4.sh
• ci: Bump to clang-8 for asan build to avoid segfaults on ppc64le
• ci: Add big endian platform - s390x
• depends: Support for S390X and POWER targets
• build: patch qt libpng to fix powerpc build

In progress

• Add POWER8 ASM for 4-way SHA256
• gitian-linux: Build binaries for 64-bit POWER
• ci: Travis support for PowerPC64
• contrib: Parse ELF directly for symbol and security checks