tccelf.c: write section headers before sections
also avoid zero lenght PT_LOAD segments (which some musl
loaders seem to dislike)
Also:
- tccasm.c: support .section with flags: .section .xyz,"wx"
(fixes
e4d874d88a2ef874a9c2fd7c47d3d3a35ae986e2)
- tccgen,c: add __builtin_unreachable()
- tccdefs.h: #define __has_attribute(x) 0
- tcc.c: tidy help for -std
- tcctools.c/execvp_win32: quote strings more correctly
- x86_64-gen.c:win32: set unwind begin-address to function-start
- github action: add aarch64-osx (M1) & i386-win32
- configure: consider 32-bit build on MSYS64 native